This paper reports the work on modeling the exhaust noise pressure of a small single-cylinder SI engine. The model will ultimately be used for on-line prediction of exhaust noise in an active control system with a single-error microphone. The frequency properties of the engine exhaust noise generated by this SI engine are characterized by harmonic components related to engine firing conditions. The frequencies of the harmonic components depend only on the engine firing frequency while the amplitudes of the harmonic components vary with the engine load. The twenty harmonic components at the lower frequency band were used to model the exhaust pressure pulse which is a two-dimensional function of engine speed and load. The model is represented by a double Chebyshev polynomial. The instantaneous exhaust pressure at steady state for a fixed position in the exhaust pipe was predicted using this model and compared with the experimental data. The result of this comparison is regarded as satisfactory.