Considering pollution problems today, investigations have been concentrated on lowering the concentration of toxic components in combustion products. A gasoline lead additive at a concentration of 0.6 g/l is used to provide a gasoline with an octane number of 98. In many countries methyl tertiary butyl ether (MTBE) is used instead of lead to produce gasoline with the same octane number. Presently, 15% by volume of MTBE is added to unleaded gasoline to provide a gasoline with an octane number of 98. However, the leaded gasoline is still used in many countries.The purpose of this research is to experimentally study the possibility of using ethanol as a lead / MTBE replacement for spark ignition engines fuels. The effect of ethanol addition to unleaded gasoline on the research octane number, performance and pollutant emission of the spark ignition engine is studied. A Ricardo E6/US engine is used in this study. Results showed that the use of 15.% by volume of ethanol is a good alternative for replacing lead or MTBE in gasoline fuel and the resulted blended fuel satisfies the global specifications. The results of the standard ASTM methods showed that with increasing the ethanol content, the research octane number of the blended fuel increases, while the Reid vapor pressure of the blended fuel initially increases to a maximum value at 10 % by volume ethanol and then decreases. The results also showed that the addition of ethanol, (from 0% to 30% by volume), increases the break power, break thermal efficiency and break specific fuel consumption, and reduces NOx, CO, and HC emissions. The higher useful compression ratio, which produced maximum break power, is directly proportional to ethanol percentage in the blended fuel.