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This paper presents some simulative and experimental results of a low emission small two-stroke cycle engine. A 23.6cc air-head stratified scavenging engine complied with CARB Tire II exhaust emission regulation was used as a base engine. A mechanism of air/fuel mixture short-circuiting cause of THC emission was investigated by numerical simulation by means of Computational Fluid Dynamics (CFD) for the base engine. To reduce the air/fuel mixture short-circuiting, port design was improved and evaluated by means of CFD. New engine performance adopted the CFD results was compared with that of the base engine. As the results of the investigation, the new engine performance at a rated speed was achieved 40% reduction of THC emission having almost same power compared with the base engine. The new 23.6cc air-head stratified scavenging two-stroke cycle engine can meet to EPA Phase2 and CARB Tire III regulations without catalyst.