The Great Dorset Steam Fair (abbreviated GDSF, and since 2010 also known as The National Heritage Show) is an annual show featuring steam-powered vehicles and machinery held in Dorset, England. It now covers 600 acres (2.4 km) and runs for five days. This used to be from the Wednesday after the UK August bank holiday, but from 2016 has been from the Thursday before the Bank Holiday until the Bank Holiday itself. It is reputedly the largest collection of steam and vintage equipment to be seen anywhere in the world.

The fair was founded by the Dorset Steam & Historic Vehicle Club, and has been held every summer since 1969. The show is now organised by Michael Oliver's son, Martin Oliver, through Great Dorset Steam Fair Ltd.

Following a meeting of like-minded individuals held in The Royal Oak in Okeford Fitzpaine, north Dorset, the first fair was held in 1969. For the first 15 years of its existence the steam fair (then known as the 'Great Working of Steam Engines') was held at Stourpaine Bushes, then in 1985 it temporarily moved to nearby Everley Hill, as Bushes Farm were delayed in harvesting the crops from the fields used by the steam fair due to the weather conditions. In 1988, after three years at Everley Hill, where access by large crowds was difficult, it moved to its current permanent home at Tarrant Hinton, north of Blandford Forum, where access is vastly improved. The fair now attracts up to 200,000 visitors. In 2020, the event was cancelled for the first time since its inception due to the COVID-19 pandemic.

The event was not held in 2023 and in November 2023, the 2024 steam fair was cancelled over rising costs.

The most numerous exhibits are traction engines, tractors and farm machinery, but there are also sections for classic cars and commercial vehicles, working shire horses, rustic crafts, 'bygones' displays, and more. The show also has a market, autojumble, live music and funfair (some of which is powered by the steam engines). The funfair has traditional rides such as gallopers and steam boats, as well as modern ones like the "World Fair Wheel" which was sited in Manchester for the millennium. It is the biggest gathering of fairground organs in the UK.

The show regularly attracts around 200,000 visitors, and there can be 30,000 people on site, making the fair the fifth largest population centre in Dorset, after Bournemouth, Poole, Weymouth and Christchurch (the population of the historic town of Dorchester being only half that number).

A speciality of the show is the display of traction engines and steam rollers performing the work for which they were designed. Such displays include heavy haulage, threshing, sawing logs, ploughing and road-making. The main arena of the show is purposely sited on the slope of a hill to allow both steam- and internal combustion-powered machinery to demonstrate their capacity for heavy load hauling. One of the main displays is the "Showman's Line up", in the vintage fairground section, which is thought to be the largest collection of showman's engines in the world. More than 60 showman's engines were present in 2004.

Since 2003, the show has contracted its own radio station, Steam Fair FM, broadcasting 24 hours daily on 87.9 FM from the Sunday prior to the show, to the Tuesday following – ten days in all. The station, which is also streamed on the internet, covers show news and views, weather and other relevant information with plenty of listener dedications and a format of "Vintage Hits". During the event, the station is advertised on roads in the surrounding area and provides traffic news for drivers using the A354 Blandford to Salisbury road that passes the show site. For the 40th anniversary, in 2008, the organisers recreated the very first fair, by tracing all of the exhibits that were displayed at the 1969 show.

2008 saw the start of the Dorset Sound Festival, a music event that is held alongside the main fair, designed to entertain a wide variety of musical tastes. The festival included five stages: the Main Stage, Real Ale Stage, Folk Stage, Country & Western Marquee and the Black Bull Marquee. Bands featured were mainly tribute acts, including the Bootleg Beatles.

In 2009, the Main Stage was changed from being inside a marquee to an outdoor concert stage. This required a concert ticket to be purchased unlike the other marquees that are free to visitors of the fair. 2009 saw the start of a new event called Steam Sounds that showcases acts from around the local area on the outdoor stage. The Main Stage is now free to all and does not require a ticket for entry as of 2014

On 31 August 2013, GDSF set a new Guinness World Record for the largest parade of steam rollers, when 103 rollers were driven into the main arena for a photo call. The previous record for moving rollers had been set by GDSF in 2003 with 32 steam rollers.

Michael Oliver is generally credited as the founder of the Great Dorset Steam Fair. A roster of participating individuals taken from the first event's souvenir program and guide in September 1969 includes:

The list is as follows;

President; Mr G. J. Romanes, M.A, M.R.C.S, D.O.M.S.

Vice President; Mr E.C. Hines.

Chairman; Mr J. E. B. Pocock

Vice Chairman; Mr M. F. Oliver

Treasurer; Mr N. J. Fincham

Public relations officer; Mr A. Imber.

Joint secretaries; Mr & Mrs J. Cluet.

Sub committee for the event;

T. M. Abbot, J. Antell, A. S. Braddick, J. Cluet, A. W. Field, G. A. Fincham, N. J. Fincham. F. Franklin, S. J. Garrett, H. Gray, E. C. Hine, A. Imber, G. J. Romanes.

Secretary M. F. Oliver.

• List of steam fairs
• Steam Era

• Official website
• Music Festival
• BBC Dorset 2008 report
• BBC Dorset report on 37th Fair (2005)