The ever increasing diesel consumption, rapid depletion of fossil fuels and environmental concern has prompted an intensive use of biodiesel produced from various sources. In this study, biodiesel (ethyl ester of fish oil) was prepared from fish oil and different blends of biodiesel-diesel (B20, B40, B60, B80 and B100) were examined in a single cylinder, constant speed, and direct injection engine under variable load conditions. On the basis of the engine performance and emission characteristics, B20 was found out to be the most favourable blend among all the blends. In addition, the test results showed a reduction of CO and HC and a marginal increase of NOx. Hence an attempt has been made to minimise NOx emission by adding little percentage (2%) of different additives such as methanol (M), ethanol (E), distilled water (DW) and diethyl ether (DEE) to the B20 blend. The results revealed that B20 blend along with distilled water produces the minimum NOx emission compared to other additives. Copyright © 2013 Inderscience Enterprises Ltd.