In this research, experiments were conducted on a 4-cylinder direct-injection diesel engine using biodiesel as an alternative fuel and their blends to investigate the emission characteristics of the engine under four engine loads (25%, 40%, 65% and 80%) at an engine speed of 1800 rev/min. A test was applied in which an engine was fueled with diesel and four different blends of diesel/biodiesel (B20, B40, B60 and B80) made from waste frying oil and the results were analyzed. The use of biodiesel resulted in lower emissions of hydrocarbon (HC) and CO and increased emissions of CO₂ and NO_x. This study showed that the exhaust emissions of diesel/biodiesel blends were lower than those of the diesel fuels.