A 125c.c. motorcycle is retrofitted with catalytic converter in order to demonstrate the emission reduction technology applied to the widely used vehicles in Taiwan. The retrofitting process minimizes the vehicle modifications while maintains the engine performance characteristics and accomplishes the maximal emission reduction results. Parameters of installation locations, substrate materials, cell density, precious metal composition/loading and catalytic converter size are all investigated in the chassis dynamometer test cell. Results of the motorcycle tested in ECE40 cycle show that significant reduction in HC emission can be achieved. But the reduction of CO emission is moderately low with the catalytic converter. The exhaust smoke is also decreased to certain extent.Use of secondary air improves the conversion efficiency of HC and CO very much effectively by supplying adequate oxygen from a reed valve system. Finally, the catalytic converter tested in engine dynamometer of 10,000km endurance cycle shows little deterioration in the conversion efficiency.