Four fire engines and around 25 firefighters 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.
The Brigade's 999 Control Officers took 12 calls to the fire.
The Brigade was called at 1837 and the ire was under control by 2259. Fire crews from Harrow, Croydon, Ruislip and Stanmore fire stations were at the scene.
The cause of the fire is under investigation.