The paper presents influence of the crankshaft offset of spark ignition engine regarding the engine friction, torque and power. It is known from the literature that crankshaft offset has a potential to reduce normal piston force during the expansion stroke at lower engine speeds. The results of lower normal force are reduced mechanical losses due to friction between the piston and liner of the cylinder. The crankshaft offset can significantly change the piston kinematics because the piston moves slower around TDC and BDC. If spark timing is referenced to the piston position from the TDC, the crankshaft offset increase in-cylinder pressure during the combustion as a result of lower change of the cylinder volume close to the TDC and engine performances can be improved. Two operating points of spark ignition engine were analyzed and optimization of crankshaft offset was performed by the NSGA-II genetic algorithm. In order to calculate the active piston force during the combustion process Vibe function were applied. The minimization of friction losses that occur between piston and cylinder liner as well as minimization of difference between peak values of resultant force of the piston in the normal direction were the main objectives of the optimization. The optimization results show that crankshaft offset has potential to improve engine performances at lower engine speeds.