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This paper describes the development of a numerical optimization technique for in-cylinder processes of SI engines, which consists of an in-cylinder processes model and an optimization model. The in-cylinder processes model simulates the full thermodynamic cycle containing spark ignition, turbulent flame propagation, heat release, nitric oxide emission and knock. The optimization model uses a nonlinear mathematical programming method, in which effective specific fuel consumption, NO emission and knock index are respectively regarded as objective function or constraint function, spark timing, compression ratio, equivalence ratio and valve timings as independent variables. The numerical optimization for a SI engine is carried out and is validated by experimental result. The engine performances are evidently improved by the optimization.