25 firefighters from Harrow, Croydon, Ruislip and Stanmore fire stations were called to a barn alight on Woodhall Road in Pinner.
An open-sided detached barn containing 200 tonnes of hay was destroyed by the blaze. There were no reports of any injuries.
Firefighters were called at just after 6:30pm and the fire was under control just before 11pm.
The cause of the fire is under investigation.
