Four fire engines and around 25 firefighters were called to a flat fire on Salisbury Road in Harrow.
A small part of a third floor flat, in a semi detached house converted into three flats, was destroyed by fire. Crews assisted six residents in evacuating the building.
The Brigade's 999 Control officers were called at 0508 and deployed crews from Harrow, Northolt and Stanmore fire stations. The incident was under control by 0616.
The cause of the fire is believed to be deliberate.