A base statement says the blaze that began at about 1 p.m. Monday has burned 120 acres and is 80 percent contained.
The fire grew quickly after the crash, growing to 50 acres three hours after the helicopter went down in the southeast corner of the base near the community of Fallbrook.
The two Marines on board the helicopter were pronounced dead at the scene. Officials say their names won't be released until their families are notified.
The cause of the crash is under investigation.