In present scenario, the fast depletion of fossil fuels and energy crises motivated the researchers towards finding an alternative sources for energy. In this work, the performance, combustion and emission of hydrothermally synthesized ZnO nanoparticles dispersed waste cooking oil biodiesel in single cylinder, four stroke, Direct Injection- Diesel Engine with eddy current dynamometer was investigated. The synthesized ZnO nano particles were characterized by XRD, FTIR, SEM and EDX to find crystallinity, functional groups and surface morphology. Waste cooking oil biodiesel was produced via conventional one step alkali catalyst Trans-esterification process. Test fuels were prepared with dispersion of ZnO nanoparticles in the concentration of 10ppm, 20ppm and 30ppm with B20 (20% waste cooking oil biodiesel and 80% diesel) with the aid of magnetic stirrer and ultra-sonication. The test results exposed that the addition of ZnO nanoparticles with B20 gave better Performance in terms of BTE and BSFC compared to B20. The addition of ZnO nano particles with 30ppm concentration was identified as better fuel among all tested fuels in this work. 30ppm addition of ZnO nano particles with B20 records 1.78 % increase in BTE and 10.34% decrease in BSFC. The emission like CO, HC and smoke were 20%, 15.4% and 17.39% lower than diesel. On the other hand NOx was slightly higher than diesel but lower than B20.