In this paper, the formation mechanisms of unburned hydrocarbons in the cylinder of spark ignition engine are investigated and the prediction model of unburned hydrocarbon formation and desorption in top land crevice, in the layers of lubricating oil on the cylinder liner and in deposits on the combustion chamber in spark ignition engines are established. The effects of different top land crevice widths, engine speeds, air fuel ratios and spark advance angles on engine exhaust unburned hydrocarbons are investigated. It is shown that the optimum width of engine top land crevice is 0.32 mm (1.3 times of the two wan quenching distance), thus the flame can get into the bottom of crevice and burns out all of the accumulated unburned hydrocarbons in it, consequently the engine exhaust hydrocarbons can be greatly reduce by 35 similar to 50% and with less penalty on engine power output and fuel economy (0.5 similar to 1.5% loss in power output and 1 similar to 3% loss in fuel economy), the predicting values by the models match well with the experimental results.