The fire was reported shortly before 2 a.m. Sunday in the 1400 block of North Sweetzer Avenue.
Los Angeles sheriff's investigators said they believe the fire was deliberately set by the victim's former roommate as a result of a prior argument. Officials said the suspect broke a front window and threw a burning can of charcoal fluid into the apartment's living room.
The woman was home alone in the four-story building when the fire broke out and was able to escape uninjured. She positively identified the person responsible as her ex-roommate. About 100 tenants were evacuated while fire crews doused the blaze in about 20 minutes.
Cade Carradine and his friend were walking by the building when the fire started. They jumped a fence and began knocking on doors trying to get people out.
"You couldn't miss it. It looked like a bomb went off," Carradine said.
No one was injured.
The top-floor unit was completely destroyed and two apartments below sustained water damage. The damage to the apartment building was estimated at $300,000.
If you have any information regarding this incident, you're urged to contact sheriff's detectives in West Hollywood at (310) 855-8850.
The Associated Press contributed to this report.