In this study, the use of waste cooking oil (WCO) methyl ester as an alternative fuel in a four-stroke turbo diesel engine with four cylinders, direct injection and 85 HP was analyzed. A test was applied in which an engine was fueled with diesel and three different blends of diesel/biodiesel (B25, B50 and B75) made from WCO. The test engine was run at 18 different speeds with a full load, and the results were analyzed. The biodiesel fuels produced slightly less smoke than the conventional diesel fuel, which could be attributed to better combustion efficiency. The use of biodiesel resulted in lower emissions of total hydrocarbon (THC) and CO, and increased emissions of NOx. This study showed that the exhaust emissions of diesel/biodiesel blends were lower than those of the diesel fuels, which indicates that biodiesel has more favorable effects on air quality.