The crash was reported at 6:14 a.m. on San Pablo Dam Road near May Road, CHP Sgt. John Carman said.
A Toyota Scion and Honda Accord collided, killing at least one person in one of the cars, Carman said.
The Contra Costa County coroner's office has responded to the scene, and the crash is blocking southbound traffic on San Pablo Dam Road, he said.
Southbound traffic is being diverted onto May Road, then onto Valley View Road and back onto San Pablo Dam Road, Carman said. He did not know when the road would be reopened.