Abstract In current work, ultrasonicator was used to produce Castor Oil Methyl Ester (COME) from raw castor oil to properly reduce the production period. GC-MS and FT-IR analyses were used to confirm the conversion of raw oil into esters. Blended fuel of COME and diesel fuel with blending ratio up to 40% (symbolized as B40) were used to experimentally investigate the influence of COME blending ratio on diesel engine combustion and on engine emissions flow as per ISO 8718 steady state test cycle. The in-cylinder pressure records were applied through zero-dimensional thermodynamic model to compute the variation of heat release rate and accumulated heat released. Results of engine measurements revealed that (i) best effective utilization of fuel energy is attained for B30, (ii) best fuel economy with best brake thermal efficiency was identified for B20, and (iii) lowest emission flow of nitrogen oxides (NOx), carbon monoxide (CO), and unburned hydrocarbons (HC) with lowest calculated particulate matter (PM) were observed for B10 while minimum opacity level in engine exhaust was attained for B30. It can be concluded that, B20 would be recommended to get the best engine mechanical performance and emission characteristics with slight deterioration in the in-cylinder combustion parameters.