It happened at the Woodland Hills Apartment Homes on West Buchanan Road in Pittsburg around 3 a.m.
Four of the 16 apartments were completely destroyed.
The public information officer for Contra Costa Fire Department said The Red Cross was on scene to help displaced residents.
CCFD said there was no sprinkler system as it was not a requirement when the building was built.
There were no injuries.
The fire is under investigation.