Biodiesel has been identified as a potential alternative fuel for CI engines because use of biodiesel can reduce petroleum diesel consumption as well as engine out emissions. Out of many biodiesel derived from various resources, biodiesel from Waste Cooking Oil (WCO) can be prepared economically using usual transesterification process. In the present study, in-depth research and comparative study of blends of biodiesel made from WCO and diesel is carried out to bring out the benefits of its extensive usage in CI engines. The experimental results of the study reveal that the WCO biodiesel has similar characteristics to that of diesel. The brake thermal efficiency, carbon monoxide, unburned hydrocarbon and smoke opacity are observed to be lower in the case of WCO biodiesel blends than diesel. On the other hand specific energy consumption and oxides of nitrogen of WCO biodiesel blends are found to be higher than diesel. In addition combustion characteristics of all biodiesel blends showed similar trends when compared to that of conventional diesel. © 2014 Production and hosting by Elsevier B.V. on behalf of Faculty of Engineering, Alexandria University.