To understand the effect of deposit in combustion chamber on engine-out hydrocarbon emissions, a two-zone model which predicts the hydrocarbon emissions contributed from combustion chamber deposits is presented and test one cylinder spark ignition engine is used to accumulate the combustion chamber deposit. The experiments are conducted on hydrocarbon emissions under various operating conditions in the presence of combustion chamber deposits. The model can successfully predict the hydrocarbons contributed from combustion chamber deposits on various engine operating conditions. The experimental results show that there exists an equilibrium thickness of deposits in combustion chamber if engine operates for a certain period of time. The combustion chamber deposits will contribute 20 ∼ 30 percent of the engine-out hydrocarbon emissions by the investigation.