The current series of experimental trial was conducted to evaluate reduction in CO, NOx and UBHC of synthetic biodiesel based on waste cooking oil at various concentrations of CeO2 nanoparticles and Ce0.5Co0.5 nano-composite oxide at full engine load. The comparative analysis indicated the significant reductions in gaseous emissions from burning of pure diesel, biodiesel and additives enriched biodiesel blends. The Ce0.5Co0.5 nano-composite oxide performance was comparatively better than CeO2 regarding the reduction of exhaust emissions resulted from the combustion of various biodiesel blends. When compared with B20 blend, the observed reduction in CO, NOx and UBHC were 18.27%, 6.57% and 23.46%, respectively by using CeO2 (100 ppm), whereas 24.18%, 13.96% and 40.74%, respectively when Ce0.5Co0.5 nano-composite oxide (100 ppm) was used as additive. Both the CeO2 and Ce0.5Co0.5 nano-composite oxide were characterized by SEM, XRD and XPS techniques. The CeO2 and Ce0.5Co0.5 nano-composite oxide may be considered as future additives in green renewable energy regime having potential to reduce CO, NOx and UBHC exhaust emissions associated with combustion of biodiesel and its blends. (C) 2019 Elsevier Ltd. All rights reserved.