Abstract: The NO_x emission is sampled near the center of exhaust port exit of each cylinder in 492QC spark ignition engine.The NO_x emission sample was analyzed using chemiluminescent detector, of which the engine speed, the spark advance and the throttle position were varied. The analyzing result indicates that NO_x emission from cylinders under the same test running condition, the maximum is over six times more than the minimum amount. Thus it is clear that the different emission amount, from cylinders shouldn't be neglected. To minimize the emission amount, especially the largest emission amount of NO_x among the cylinders are of considerable significance in reducing exhaust emission from engine.