Flames broke out around 2:30 p.m. Wednesday at the south end of Lake Berryessa in the Cold Canyon area.
Cal Fire says it brought in six air tankers, 16 truck companies, 11 bulldozers and five water tenders to try to slow down the blaze, but winds in the area are making the firefight difficult. As of 4 a.m. Thursday, the blaze was 5 percent contained.
More than 500 fire personnel are on scene, some coming from as far away as San Diego.
Campers at Canyon Creek Campground and hikers in the Cold Canyon area of Napa County were evacuated. The communities of Golden Bear and Quail Ridge were also under evacuation.
Fire officials say one home and one outbuilding were damaged and 200 structures are threatened. No injuries have been reported.
PHOTOS: Large fire burns south of Lake Berryessa
KGO-TV and The Associated Press contributed to this report.