A significant amount of waste cooking oil (WCO) remains in the food preparation process worldwide on a daily basis, which can cause environmental pollution if disposed of improperly. The cheapest and effective way to dispose of WCO is by blending it with diesel fuel. In this study three various blends of WCO and diesel fuel were prepared with a WCO proportion of 10%, 20% and 30% and properties were evaluated and compared with petroleum diesel fuel. The density and dynamic viscosity of the blend increase with the increase in WCO content, while the heating value decreases. The blends were then tested in tractor direct injection diesel engine to determine the effect of blending WCO with diesel fuel on the engine performances (power, torque and fuel consumption) and exhaust gases emission (CO, CO₂, HC and NO_x). The results show that the engine performances using WCO blends are comparable to petroleum diesel fuel and a blend with 30% WCO (B30) achieved the best results among the fuel blends. There were no significant differences in average engine power, torque and fuel consumption per hour between petroleum diesel fuel and B30 blend, while significantly lower (P˂0.05) average specific fuel consumption were achieved using diesel fuel. The significantly lower (P˂0.05) average CO and CO₂ emissions and significantly higher (P˂0.05) average HC emission was achieved using diesel fuel compared to B30 blend. No significant difference in average NO_x emission was observed between diesel fuel and B30 blend.