This research was proposed to investigate the utilization of used frying oil as an alternative fuel for diesel engines. Edible oils that have been used several times for frying purposes usually contain undesirable components that make them unsuitable for further use for human consumption.The frying oil used in this research was chemically modified to reduce its viscosity by thermal cracking in the presence of 2% calcium oxide as a catalyst. The product was then evaluated as diesel engine fuel according to its fuel properties and the performance of an engine fueled with its blend with diesel fuel (50% blend).The results have shown that the product obtained by thermal cracking of frying oils has fuel properties similar to those of diesel fuel. Also, the performance of a diesel engine in terms of output power, brake thermal efficiency, and brake specific fuel consumption have not been markedly changed by blending diesel oil with the cracked product (50% blend). Moreover, the cracked product was free from sulfur and nonburnable matters. This means there are 3 major advantages over standard diesel oil. These include less corrosion of injection parts by acidic sulfur oxides, less wear of the engine parts by ash deposits, and less environmental pollution by the combustion exhaust.