This work presents the numerical simulation of a compression ignition engine, fueled with diesel oil at 25°C and straight soybean oil / diesel oil mixture (80% and 20% respectively) at 85°C. Variables as discharge coeficient of the inlet valve, volumetric eficiency, velocity and pressure fields in the cylinder, swirl, tumble, liquid penetration, vapor penetration and drop diameter were obtained and analyzed for both the fuels. CONVERGE CFD™ software was used with the Reynolds Average Navier-Stokes equations with the k -e Renormalization Group turbulence model. For the spray behavior simulation were employed the Blob and Kelvin Helmontz-Rayleight Taylor (KH-RT) models for primary and secondary breakup, respectively. Fuel's physicalchemical properties were measured or considered according to information reported by other authors. Tests of the engine on dynamometric bench, allowed obtaining the average in-cylinder pressure as a function of the crank angle. These experimental results and the information from other similar studies found in the literature were used to validate the numerical model. Experimental and numerical results for the two types of fuels showed good agreement which ensures that the methodology and models used in the numerical simulation of the compression ignition engine is suitable also for straight soybean oil / diesel oil mixtures analysis. In addition, it was observed that the mixture presents larger droplet size, higher spray penetration and a lower evaporation rate when compared to diesel.