Abstract: The effect of methyl tert butyl ether (MTBE) addition into gasoline on the exhaust emissions from EFI gasoline engine was studied. A four cylinder HL495IQ engine was used in the experiments. Fuels containing 0,5%,10%,15%,20% MTBE were used in the engine operations, and the exhaust gases were analyzed for CO, HC and NOx, before and after their catalytic treatment by a threeway catalytic converter. The addition of MTBE into gasoline resulted in a decrease in CO, THC and NOx emissions only at low speed or light loading. No obvious effect were observed at other conditions. Catalytic conversion efficiencies of CO and THC were inproved to 70% and 99% respectively. But the catalytic conversion rate of NOx only reached 12%~55%. The efficiency of catalytic converter had relations to speed, load of the engine and the MTBE content in the blended fuels.