A new estimator for A/F ratio in SI engines is described. A considerable fraction of the HC emissions from internal combustion engines are generated just after the engine is started. HEGO oxygen sensor cannot measure the A/F ratio of exhaust gas during cold start condition. The A/F ratio estimator described in this study is based on the measurement of the exhaust gas temperature, air flowrate, and coolant temperature, etc. A/F ratio can be estimated indirectly in that the exhaust gas temperature relies on A/F ratio of Exhaust gas. The HC emissions are also measured during the cold start condition. The Generalized Regression Neural Network Function Approximation(GRNN) is used to estimate the A/F ratio of exhaust gas.