The thirst for fuel is steadily increasing as technology continues to open new areas of exploration. At the same time, the indiscriminate extraction of fossil fuels also may result in extinction of petroleum deposits in foreseeable future. Along with this, pollutant emission from diesel engines causes major impacts on ecological systems. In order to overcome the above problems associated with the use of petroleum derived fuels, a suitable source of biodiesel should be used to replace conventional diesel fuel. Hence, in this work, feasibility of using biodiesel prepared from fish oil was investigated. Various properties such as viscosity, density, calorific value, flash point and cetane value of biodiesel and biodiesel-diesel blends of different proportions were investigated. Later, experimental tests were carried out to evaluate the performance, emission and combustion characteristics of a single cylinder, constant speed, direct injection diesel engine using biodiesel-diesel blends, under variable load conditions. It was found that there was a reduction in NOx, HC and CO emission along with a marginal increase of CO2 and smoke emissions with the increase in biodiesel proportion in the fuel. The brake thermal efficiency was found to be higher compared to diesel for the entire load. An analysis of the cylinder pressure rise, heat release, and other combustion parameters such as peak pressure, rate of pressure rise, combustion duration and ignition delay was carried out. The ignition delay, maximum heat release rate and combustion duration were lower for biodiesel-diesel blends compared to diesel. Ultimately, fish oil can indeed become the appropriate source for biodiesel, with environmental benefits. (c) 2014 Elsevier Ltd. All rights reserved.