The fire happened behind Al's Ricos Tacos on Ventura Avenue and Hazelwood Boulevard just after 11:00 pm.
Firefighters say the flames sparked outside a detached building that stored appliances and boxes for the restaurant. The blaze then spread inside the unit.
The building was destroyed.
"We had to do a search in case anybody was in it. The fire was basically under control in a few minutes. We protected a residential house just south of the main fire," said Fresno Fire Battalion Chief Tony Escobedo.
Firefighters say Al's Ricos Tacos is still intact and was not damaged by the fire.