The article presents the laboratory tests results of the influence of the diesel oil content in a mixture with rapeseed oil on the operating parameters of a diesel engine with the CommonRail fuel supply system. Fuel mixtures with a rapeseed content of 0%, 50%, 70%, 100% were used. The article presents the results of tests carried out for various configurations of the object. The measurements included tests of exhaust gases: the content of carbon monoxide, carbon dioxide, hydrocarbons, nitrogen oxides as well as specific fuel consumption and exhaust gas temperature. As a result of the research, it was noticed that the use of rapeseed oil causes a significant increase in fuel consumption and an increase in emissions. At a crankshaft speed of 2500 rpm and a torque generated by the engine crankshaft of 12 Nm, the specific fuel consumption for a 0% rapeseed content is 310 g/kWh. For the same engine operating conditions and the same rapeseed content of the fuel, the fuel consumption for 100% rapeseed content is 330 g/kWh. For the same engine operating parameters, analysis of the carbon dioxide content in the exhaust gas yields a value of 7.8x104 ppm for a rapeseed content of 0%. For the same engine operating conditions and for the oilseed rape content in the fuel is 100% 8.4x104 ppm. The mathematical models describing the effects of engine operating parameters and rapeseed oil content in the mixture on the observed values were created with the Scilab program. The degree of agreement of the determined mathematical models was checked by calculating the coefficient of determination (R2). The results are shown in 3D figures.