The Chino Valley Fire Department said the Grand Fire, which ignited Wednesday night in Chino Hills, had burned about 36 acres near the border with Diamond Bar as of 7 p.m. Thursday.
Officials said the fire is 75% contained as firefighters worked to improve containment t lines. Crews were expected to remain on scene overnight.
Grand Avenue reopened to all traffic at 7 p.m. Thursday.
Firefighters say cooler temperatures and increased humidity, helped them get the upper hand.
The fire initially sparked evacuation orders and warnings for nearby residents, but they have since been lifted.
One firefighter was reportedly transported to a local hospital with a minor injury.