This article investigates the effects of using diesel-biodiesel blends on compression ignition engine performance characteristics. Combustion stability observed from in-cylinder pressure related parameters, biodiesel composition, and the fuel properties of the blends are used to analyze the variations of the characteristics. Making biodiesel from hazelnut oil is done by esterification and then experiments are conducted with the neat diesel and diesel-biodiesel blends in a four stroke naturally aspirated direct injection diesel engine. Volumetric blending ratios of biodiesel with no. 1 diesel were set at 0, 20, 40, 60, 80, and 100. It has been found that, in general, the hazel nut oil-based biodiesels behave comparably to the diesel fuel in terms of engine performance characteristics. There is no significant brake torque and power reduction, and there is a slight increase in brake specific fuel consumption when the engine is operated with diesel-biodiesel blends. The engine effective efficiency shows improvements as compared to that of the diesel fuel operating under similar conditions, especially at low blending ratios of biodiesel. The engine performance characteristics and exhaust emissions improve remarkably at B20 operations and identifying the mechanism for biodiesel blending effects is attempted.