Internal combustion (IC) engines increase the growth of our society and civilization. This research focuses to investigate the emission, combustion and performance behaviour of single cylinder DI diesel engine fuelled by diesel/watermelon seed oil biodiesel (WMB) blends. The oil from watermelon seed was converted into biodiesel (methyl-ester) through Trans-esterification process. The watermelon seed biodiesel was blended with at different proportions to diesel fuel at 20, 30 and 40%. Among the blends B20 gave better performance, improved CO and HC emission in all aspects except NOx emission. The BSFC of B20 is 0.275 kg/kWh and BTE is 32.5% at full load these are on par with diesel. Therefore, B20 of watermelon seed oil biodiesel as a best alternative fuel in coming years. © 2021 Elsevier Ltd. All rights reserved.