CASTOR oil as well as used cooking oil have been catalytically cracked using three types of catalysts being zinc chloride, sodium carbonate and calcium oxide and the products of cracking were tested for their chemical and physical properties relevant to their use as fuels. Products obtained by cracking castor oil were found to be more suitable as bio-kerosene while those obtained by catalytic cracking of used cooking oil were more suitable as biodiesel. Therefore, the product obtained by catalytic cracking of used cooking oil has been then tested for its effect on the performance of a diesel engine compared to regular diesel fuel. Blends of catalytically cracked used cooking oil with regular diesel fuel were used for running the engine at different engine loading and the specific fuel consumption as well as the exhaust temperature had been determined at each load. The results have shown that blending of regular diesel fuel with catalytically cracked oil resulted in a reduction in the thermal efficiency compared to that if the engine was run using regular diesel fuel only .Such blending resulted also an increase in the exhaust temperature.