The bus was forced to pull over on the 10000 block of East Imperial Highway.
Police say about 40 people onboard were traveling back to Compton from a Pasadena movie theater when they smelled a suspicious odor or smoke. That's when the driver exited the freeway and pulled into a doughnut shop parking lot where the engine caught fire.
Investigators say a mechanical failure caused the fire.
No injuries were reported.
The Associated Press contributed to this report.