In the development of the OBD system of a vehicle, an aged catalyst is required to test the function of the system. This catalyst is partially deteriorated and the emission after this catalyst will exceed the regulation value with limited bound according to the testing protocol. This aged catalyst is very valuable, and difficult to prepare. A bypass in the exhaust pipe was used to simulate the deterioration of catalyst in this paper to investigate the emission characteristics of a motorcycle engine. The degree of deterioration can be controlled by the portion of exhaust flow through the parallel pipe.This method of simulation has been conducted in a dynamometer testing. An electronic controlled motorcycle engine was used in this test. It was found that at low load condition, the CO and HC concentrations in the downstream of exhaust pipe were increased as the control valve was opened to let more and more exhaust to flow through the straight pipe. The increment was proportional to the flow that has been detoured. However, at high load condition, the variations of the exhaust emission seemed to be not relevant to the opening of bypass valve due to the complicated flow dynamics in pipes.