Four fire engines and around 25 firefighters were called to a fire at a flat on Glenloch Road in Belsize Park.
Part of a first floor flat in a five storey block was damaged by the fire. One woman left the affected flat before the Brigade arrived. She was assessed at the scene for smoke inhalation.
The Brigade's 999 Control Officers took 10 calls to the blaze.
The Brigade was called at 1316 and the fire is under control by 1428. Fire crews from Kentish Town, West Hampstead, Euston and Paddington fire stations were at the scene.
The cause of the fire is under investigation.