In this study, energy performance of diesel engine experimentally examined with cooking oil – diesel blends (D95B5, D90B10, D85B15 and D80B20) as retrofit for pure diesel (D100). An assessment of engine performance was investigated at varying engine speeds load conditions for both pure diesel and its blends at both laboratory conditions and under British Standard (BS) conditions. Results of this investigation show that the average engine torque reduction compared to D100 for D95B5, D85B15, D90B10 and D80B20 was found as 9.03%, 10.32%, 13.95% and 23.06% and average engine power reduction was found as 7.09%, 11.48%, 12.43% and 21.12% respectively. In terms of bte, average increase in bte was found as 5.95% and 0.29% for D95B5 and D85B15 but average reduction was found as 3.17% and 9.45% for D90B10 and D80B20 fuel blends, respectively. While, D95B5 was 5.32% lower than D100. From this study, D95B5 produced better performance when compared to D85B15.