This book is a comparative study of the evaluated biodiesel derived from waste vegetable oil and fossil diesel fuel in the running of diesel engine. The main objective of the research project was to add value to waste vegetable oil by converting to biodiesel for use as diesel fuel supplement and help reduce the cost of petroleum based fuel which is fast depleting. Biodiesel fuel was evaluated on the short-term performance of the selected diesel engine. The operating conditions for the modified reactor were 60L vegetable oil, 10L methanol at a temperature of 65-68 degrees centigrade. The time of reaction was 3hours. The catalyst used was 250g anhydrous Sodium hydroxide. The diesel engine operating combustion conditions were obtained at engine speed rating of 1750 rpm for both fossil diesel and waste vegetable oil derived biodiesel. The average fuel consumption was 0.3396 kg/KWh for fossil diesel fuel and 0.3804 kg/KWh for biodiesel respectively. There was no significant difference in the variation of fuel consumption and thermal efficiency for both fossil and biodiesel fuels.