Biodiesel has a vital role in recent years as an alternative fuel for diesel engines. Biodiesel was produced from waste cooking oil by transesterification process. Physical and chemical properties of biodiesel blends up to 20% were near to diesel fuel. The aim of this study to is to reuse biodiesel produced from waste cooking oil as a fuel and make a comparative study of performance parameters and exhaust emissions between biodiesel blends. Diesel- waste cooking oil biodiesel blends of 10 and 20% were prepared. Experimental investigations were tested in a four stroke, single cylinder, diesel engine at a constant speed of 1500 rpm and variable loads. Diesel- biodiesel blends provided significant increase in fuel consumption, specific fuel consumption, exhaust gas temperature, NOx, CO2 and oxygen concentrations about diesel fuel. Waste cooking oil biodiesel blends resulted in significant decrease in thermal efficiency, volumetric efficiency, air- fuel ratio, CO and HC emissions about diesel fuel. Performance characteristics and exhaust emissions concentrations for biodiesel blends were near to diesel fuel, so, we recommend using biodiesel blends up to 20% in diesel engines without engine modifications.