A Mandurah Bus Charters bus has burst into flames at the side of the road on Kwinana Freeway in Baldivis near the Mundijong exit.
The bus was reported by passersby at 6.55am, with the incident causing traffic blockages and cars backed up along a wide stretch of the Freeway.
A representative from WA Police Media told the Mail that the bus had one occupant who escaped unharmed.
The fire has been extinguished, but the scene continues to cause traffic disturbances.
A Department of Fire and Emergency Services spokeswoman said the accident was believed to be accidental, originating in the engine compartment.
"It is up to the bus company to determine if they need to do anything further for insurance purposes," the spokeswoman said.
Mandurah Bus Charters has been contacted for comment.