Biodiesel is an alternative fuel for diesel engine. Considering the differences between diesel and biodiesel fuels, engine condition should be modified based on the fuel or fuel blends to achieve optimum performance. In this study, the effect of fuel injection pressure to inject fuel in to combustion chamber on brake thermal efficiency of a direct injection diesel engine was investigated experimentally. Different experiments were carried out on a diesel engine using different blends of diesel and biodiesel fuels choosing speeds of 1600, 2000, 2400 and 2800 rpm. During each experiment, with an eddy current dynamometer the brake torque and with a fuel consumption measuring system, the fuel mass flow was recorded. During experiments, the fuel injection pressure into cylinder was regulated on 185, 200 and 215 bar. The results of brake thermal efficiency for fuel blends of B20, B40, and B100 in different engine speeds and injection pressures were recorded and compared. The results of this research work showed that increasing fuel injection pressure for fuel blends of B20, B40, and B100 increased brake thermal efficiency approximately 2.1%, 2.3% and 2% respectively and decreased maximum brake specific fuel consumption approximately 3%, 3.9% and 2.6% respectively.