Biodiesel, an alternative fuel is derived from the fats of animals and plants. As energy demands increase and fossil fuels are limited, research is directed towards alternative renewable fuels. The main advantages of using this alternative fuel are its renewability, biodegradability and better quality of exhaust gases. It is technically competitive and environmentally friendly alternative to conventional petro-diesel fuel for use in Compression Ignition (CI) engines. The use of biodiesel reduces the dependence on imported fossil fuels which continue to decrease in availability and affordability. An experimental investigation has been carried out to evaluate the combustion, performance and emission characteristics of a diesel engine with the effect of using neem oil methyl ester and its diesel blends at different loads. The results showed that maximum cylinder pressure and maximum rate of heat release increased with the increase in biodiesel blends. The carbon monoxide (CO) and smoke emissions were found significantly lower when operating on biodiesel-diesel blends, but Nitrogen Oxide (NOx) emissions are found to be higher at full load. © 2013 Science Publication. © 2013 Elsevier B.V., All rights reserved.