At 6:16 a.m. firefighters responded to the fire at a townhouse in the 20000 block of San Miguel Avenue, according to Alameda County Fire Department spokeswoman Aisha Knowles.
The victim was identified as Jacqueline Burns, 51.
The fire occurred about 150 feet away from an Alameda County Fire Department fire station.
"I opened door and looked over there and the flames must have been 30 feet in the air," neighbor Bill Kruisink said.
Kruisink saw the smoke and fire engulfing the second floor of the duplex.
"When we opened the apparatus doors up to respond to the fire, we could smell the smoke," Alameda County Fire Department Batt. Chief Ronnie Otis said.
Crews say they responded in under a minute, put out the fire, but later located the body of a woman in an upstairs bathroom.
"The rescue company pulled the victim out and we tried to revive the victim, but were unsuccessful," Otis said.
A neighbor who didn't want to go on camera said she heard the woman screaming. "I went back out to put the water hose on to the back of my house and when I did that I heard screaming from the bathroom, and it was a woman's voice," one woman said.
The fire department is still investigating a cause, but did confirm there were working smoke detectors in the duplex.
Earlier this week, a body of a woman was found in her Castro Valley home on Grove Way after a fire broke out there on December 13.
Police say she had been murdered, and the fire was an attempt to cover up the crime. So far, no arrests have been made.
Investigator say the two fires are unrelated.
The identity of the victim has not been released and a cause for the fire remains under investigation.
Bay City News contributed to this story.