Fuel quality is considered an important indicator for ensuring maximum output power. Hence, using commercial fuel that didn’t meet the standard specifications may result in engine performance deterioration. In this study, Methyl tert-butyl ether additive has been introduced as an octane enhancer with local low-octane gasoline. Five samples were prepared by adding MTBE additive at 3%, 6%, 9%, 12% and 15% to commercial gasoline and tested at constant half load and increasing engine speed. Response surface method optimization and ANOVA analysis have been conducted using the obtained engine test results to indicate the optimum engine performance conditions and the significance of the variations respectively. Optimization results show that the output responses are linked statistically with the engine speed. The optimum operation conditions obtained at an engine speed of 3126 rpm and 3% additive ratio by 2356 W for brake power, 0.187857 g/W.hr for brake-specific fuel consumption and 38.3% for the brake thermal efficiency. According to the chosen significant level, ANOVA results shows significant influence of engine speed on the different obtained responses, On the other hand, the optimum response achieved at 3% additive ratio with insignificant influence for increasing additive ratio in the fuel mix.