Abstract: To simulate the combustion in the cylinder of gasoline engine with steam reforming, a detailed mechanism was developed. The mechanism of GRI 3.0 and gasoline surrogate were used for the construction of the detailed mechanism according to the components in syngas. The detailed mechanism, with 334 species and 1672 reactions, was reduced with direct relation graph method to form a reduced mechanism with 40 species and 171 reactions. The reduced mechanism was verified by the one dimensional gasoline engine model by the test of the average temperature, the temperature error, the heat release rate, the heat release rate error and the difference between OH concentration. Results show good agreement of the validation of detailed mechanism and reduced mechanism.