A single-cylinder optical research engine for combustion visualization has been developed by modifying a 4-cylinder DOHC, spark-ignition engine. The design tips for the optical engine development are described and the visualized flame images are given. The optical engine has a piston-top window in the extended piston which allows optical access into the combustion chamber for flow and combustion research. The flame propagation at 800rpm was imaged by an ICCD camera for stoicheometric mixture under idling condition.