Vegetable oil based fuels are promising substitutes for diesel fuel for transport and industrial applications. Converting raw vegetable oils into their ethyl or methyl esters has reasonably solved the problems associated with their high viscosities. The purpose of the research is to investigate experimentally the effect of injection pressure on the combustion characteristics of a biodiesel engine using cerium oxide nanoparticles as a fuel additive. Biodiesel derived from waste cooking oil mixed with mineral diesel in the proportion 20:80 is used along with cerium oxide (CeO2) nanoparticles as additives. The engine is run at several fuel injection pressures (180, 210 and 240 bar) and with a nanoparticle concentration of 80 ppm. The experiments demonstrate that the higher injection pressure associated with nanoparticle addition improves engine combustion characteristics, e.g., it leads to a higher peak pressure and a higher heat release rate. The best performance was observed at an injection pressure of 240 bar with a 80 ppm nanoparticle concentration. Concentrations of hydrocarbon, oxides of nitrogen and smoke decreased for higher injection pressures with nanoparticle addition. Hence an enhancement in engine performance and decrease in emissions can be achieved in a biodiesel engine by employing higher injection pressures with nanometer sized cerium oxide particles as additives. This helps in conserving the petroleum based fuels and also avoiding the emissions of contaminants to the atmosphere. (C) 2019 Elsevier Ltd. All rights reserved.