An experimental investigation has been carried out to evaluate the effect of using diethyl ether as additive to biodiesel on the combustion, performance and emission characteristics in an unmodified diesel engine at different loads and constant engine speed. The results indicate that peak cylinder pressure and heat release rate is higher for BD5 (5% (by vol.) diethyl ether blended biodiesel) than those of neat biodiesel. The carbon monoxide emissions especially at full load and smoke emissions at almost all engine loads decrease while oxides of nitrogen and hydrocarbon emissions increase for BD5 than those of neat biodiesel at almost all engine loads. The brake thermal efficiency of BD5 is higher as compared to biodiesel. (C) 2012 Elsevier Ltd. All rights reserved.