The fire was reported around 5 a.m. on Harbor Boulevard, according to the Orange County Fire Authority.
AIR7 HD was over the scene as firefighters attacked the flames with water cannons on top of four large ladders. The fire agency later tweeted that crews were making good progress.
The building housed several businesses, including one that sells batteries for hybrid and electric vehicles and some sort of restaurant.
The cause of the fire is unknown. No injuries were reported.