The fire happened around 1:15 p.m. at Maple and Andrews.
Firefighters were able to contain the fire to just one bedroom of the unit.
They say a mom and her five children made it out safely thanks to a smoke detector.
Smoke made its way through the entire apartment so everything inside has been destroyed.
"The fire is under investigation. Our investigator is talking to the family as well as looking through the burned bedroom looking for any clues as to the cause of this fire," Fresno Fire Battalion Chief Casey Clark said.
Two young children, 6 and 3 years old, were in the bedroom when the fire started, but the cause is still not known.
Fire officials say even though there was at least one working smoke detector they did find one with no battery.
They say this is a perfect reminder to check the smoke detectors in your own home. The Red Cross was helping the family.