Combustion rates of blends between plant oils and diesel oil were predicted by a convective mass transfer theory. Test results for the combustion rates of diesel oil blended with palm oil, soy bean oil and rice bran oil, on a spherical model of 50 mm diameter, agree well with the predicted values. The theory was extended to predict performances of a C.I. engine run on blends between the plant oils and diesel oil. Correction factors for engine friction were incorporated in the prediction. Test results for the engine performances agree fairly well with the predicted values. Amounts of carbon monoxide in the engine emission were measured and found to increase with the increasing percentage of the plant oils in the blends.