In this study, the optimum blend rate and injection pressure in a four-stroke, single cylinder, direct injection diesel engine using soybean methyl ester were investigated experimentally. The tests were conducted at two stages. Firstly, the engine was tested with diesel fuel, B25 (25% biodiesel + 75% diesel fuel), B50, B75, and B100 fuels at full load and at a constant speed. According to the test results, it was determined that the most suitable fuel was B25 in terms of performance and emission. Secondly, the engine was tested at different loads with diesel fuel at original injection pressure and with B25 at different injection pressures (160, 180, 200, 220, and 240 bar) for comparison. It was determined from tests performed with B25 that the most suitable injection pressure in terms of performance and emissions was 220 bar. The specific fuel consumption and power values of the B25 were found to be nearly the same as those of diesel fuel at 220 bar injection pressure. In addition, HC, CO, and smoke emissions were reduced by about 33%, 9%, and 20%, respectively. On the other hand, NO emission increased by about 12%.