The behaviors of a biofuel and a fossil fuel were evaluated by different techniques, namely thermal analysis (Thermogravimetry-TG and Differential Thermal Analysis-DTA) and engine performance tests. From TG and DTA curves, two decomposition phases were identified for diesel fuel with ignition temperature (IT) at 250 degrees C and three phases for biodiesel fuel with IT at 300 degrees C. Combining conversion (a) versus temperature and a model free kinetics method, the range of activation energies (Ea) values were determined for both diesel (from 48.5 kJ mol(-1) up to 61.0 kJ mol(-1)) and biodiesel (from 58.6 kJ mol(-1) up to 55.0 kJ mol(-1)) fuels. At the initial phase of combustion, diesel fuel presents lower E-a and IT than biodiesel fuel. At the final phase of combustion, an opposite behavior is observed. These results provide subsides to predict that the behavior of diesel-biodiesel blends is the best option due to the lowering of E-a along the entire combustion process. As regards the engine tests, the B20 blend showed improvements compared with diesel fuel. In average, B20 presented increases of 1.2% in power, 1.0% in torque and 1.2% in thermal efficiency. In terms of soot, B20 presented 8.9% lower emissions than diesel fuel. Although thermal analysis and engine performance tests are executed under very different conditions, the results from both techniques showed the same trend, i.e., the best option for the combustion performance is a diesel-biodiesel blend.