UPDATE: A semi-trailer was on fire and a truck rolled after two trucks crashed on Old Coast Road in Binningup just before noon.
The RAC Rescue helicopter took an injured man to Royal Perth Hospital.
Water bombers were sent to the area after flames spread into nearby bush.
Early reports said that one of the trucks was carrying paint, which may have been a hazard.
Motorists were urged to take care in the area, which is near Binningup Road, about 20km north of Bunbury.
Career and volunteer Fire and Rescue Service and Bush Fire Service firefighters helped a St John Ambulance crew and contained the small bushfire.