The oxidation of unburned hydrocarbons from piston crevices was modeled using a modified one-step reaction equation. This new one-step oxidation model was developed by modifying the Arrhenius reaction rate coefficients of the conventional one-step reaction equation. The predictions of the new one-step oxidation model agree well with the results of the detailed chemical reaction mechanism in terms of the 90% oxidation time of the fuel. The effects of pressure and intermediate species in the burnt gas on the oxidation rate were also investigated and included as additional multiplying factors in the modification of the equation. To simulate the oxidation process of unburned hydrocarbons from a piston crevice, a twodimensional computational mesh, based on the conventional engine geometry, was constructed with a fine mesh density at the regions of the piston crevice and cylinder wall. The number of cell layers in the cylinder was controlled according to the piston motion to model the out-flow of unburned hydrocarbons from the piston crevice during the expansion stroke. The effects of engine operational conditions on the oxidation rate were examined at several engine speeds and load conditions, and the sensitivity of the oxidation rate to the piston crevice volume was also evaluated. Finally, the new one-step oxidation model was applied to a three-dimensional computational mesh that modeled the three-dimensional engine geometry and piston-valve motions to simulate the oxidation of unburned hydrocarbons in a real engine condition.