A three-dimensional model for oxidation of unburned hydrocarbons at the SI engine exhaust port has been developed. The computational mesh with a moving grid for the valve motion is constructed for the port geometry of a single-cylinder research engine. A 4-step oxidation model is used to predict the HC oxidation and the coefficients of the model are modified for the port oxidation environment with the results of a full chemical kinetic mechanism. The measured THC concentrations at the cylinder exit (3 positions) are used as the inlet concentration for the simulation and the temperatures of the core gases and boundary zone gases in the cylinder are obtained from a three-zone cycle simulation. The simulation result shows that the degree of the THC oxidation in the exhaust port at 1,000 rpm, IMEP 550 kPa, stoichiometic condition is 31.5%. The effects of inlet gas temperature, oxygen concentration, heat transfer, and engine operating conditions on the THC oxidation have been investigated. THC oxidation in the exhaust port is predicted to be between 14% and 46% depending on the engine operating conditions. The simulation results are in good agreement with other research results.