A conventional automotive emission control system depends on the measurements provided by various sensors to control the air-fuel (A/F) ratio. Maintaining the A/F ratio close to stoichiometry permits catalytic converter to operate in an optimized efficiency, which reduces the exhaust emission. Malfunction resulted from engine misfire makes catalyst''s converting efficiency drop. Such a condition results in increased emissions as well as in damage to catalytic converters. So current researches are proceeded in response to the California OBD II (On-Board Diagnostics) and EOBD that will be adopted in Europe requirements for engine misfire detection in passenger vehicles.In this study, two methods to diagnose the misfire an approached: catalytic converter''s temperature measurement over the threshold exposure temperature to examine the catalyst''s damage, and the vehicle emission test over FTP-75 cycle by varying misfire rates. The emission performance of catalytic converter and the catalyst exothermal reaction has been investigated. The relation between misfiring rate and conversion performance of catalytic converter has analyzed, which improved the understanding of the vehicle emission characteristic of OBD II system.