The growth of energy demand and limited fossil fuel resources lead to renewable energy development such as vegetable oils and animal fats or their derivatives. In the present work, the valuation of waste fish fat by the pyrolysis technique with the presence of catalyst to produce biofuel for diesel engines. As a result, fuel undergoes good combustion and hence there is a significant improvement in performance and reduction in emissions. The brake thermal efficiency of neat biofuel is 32.4% at 80% load which is very high compared to neat diesel (29.98%). The combustion duration and ignition delay are decreased with neat biofuel due to high oxygen content and high cetane number of biofuel. The main problem with the use of neat biofuel in diesel engine is high NOx emissions at all loads. Addition of diesel with biofuel reduces the NOx emissions significantly from 917 ppm to 889 ppm at 80% load with an optimum blend of B80D20. There is a slight decrease in brake thermal efficiency and increase in particulate emission with this blend. The overall results show that by adding small quantity of diesel with biofuel decreases the NOx emissions significantly and approaches the performance of neat biofuel. (c) 2012 Elsevier Ltd. All rights reserved.