Six fire engines and 35 firefighters and officers were called to a fire at a restaurant in Finchley Road in Swiss Cottage.
The restaurant's ducting from the basement to the roof was destroyed in the fire. An adjoining restaurant also suffered fire damage to its ducting system.
Staff in the restaurant where the fire originated left the building before the arrival of the Brigade. There were no reports of any injuries.
The Brigade was called at 1216 and the fire was under control at 1407. Fire crews from West Hampstead, Euston, Paddington, North Kensington and Soho fire stations attended the scene.
The cause of the fire is believed to be a burning ember from charcoal cooking in the kitchen igniting a build up of grease within the ducting.