In this study, a conventional compression ignition (CI) engine is modified to operate in dual fuel mode with biogas and diesel as the main and pilot fuels respectively. The effect of biogas flow rate on engine performance was studied. The viability of using cerium oxide (CeO2) nanoparticles as a diesel additive was also investigated. Compared to diesel-only mode, dual fuel mode with biogas flow rates up to 8 L/min was observed to improve the brake thermal efficiency at high loads. Higher biogas flow rates provided significant diesel substitution. Biogas was observed to contribute up to 80% of the total energy release. Volumetric efficiency deteriorated with increase in biogas flow rate, while overall equivalence ratio and exhaust gas temperature increased. After addition of CeO2nanoparticles, various properties of modified diesel were measured and tabulated. Performance tests revealed that modified diesel containing 25 mg/L CeO2marginally improves brake thermal efficiency. In general, the addition of CeO2increases the overall equivalence ratio and exhaust gas temperature. Tests were carried out for all combinations of biogas flow rate and CeO2concentrations. Dual fuel operation with 4 L/min biogas flow rate and 25 mg/L CeO2concentration was found to provide the highest brake thermal efficiency. © 2016 Informa UK Limited, trading as Taylor & Francis Group.