Use of catalyst in engines has entailed a radical increase in the importance of misfire detection. When a misfire occurs, hydrocarbon emissions will increase and the unburned fuel can damage the catalyst by overheating. On-Board Diagnostics II (OBDII) regulations are still not applied to motorcycle or moped yet. However its application is under discussion in European Union. In Taiwan, OBD is scheduled to be implemented soon. Many strategies of misfire detection have been developed, including variation in engine shaft angular speed, spark plug voltage, cylinder pressure, oxygen sensor signal, knowledge based expert system, and neural networks. WE propose a new method to use the real time signals of a wide band oxygen sensor to detect misfire where, misfire was induced on purpose with a misfire generator. The sensor and the misfire trigger signals were recorded simultaneously. It was found that when a misfire occurs, a spike of sensor signal would follow due to instantaneous variation of the oxygen content in the exhaust flow. This signal was then processed to detect the occurrence of misfire. Results of test show that differentiated signals have good correlation with the misfire trigger. Misfire is detected by monitoring the amplitude of differentiated signals. The scheme has been shown to detect the misfiring condition reliably up to 5000rpm.