Four fire engines and 21 firefighters and officers were called to a fire at a four-storey block of flats in Tyrrel Way, Colindale.
Half of a three-roomed flat on the ground floor and part of the corridor were damaged by fire.
One resident left the flat before the arrival of the Brigade and another 39 people evacuated the block. There were no reports of any injuries.
The Brigade was called at 0704 and the fire was under control at 0746. Fire crews from Hendon, Stanmore and West Hampstead fire stations attended.
The cause of the fire is being investigated by the Brigade and the Metropolitan Police Service.