Abstract: The phase characteristic of simple harmonic components of torsional vibrations of internal combustion engines was studied by simulation. The calculate results of torsional vibrations influenced by the phases of simple harmonic actuating torque was analyzed. The relationship between the phase characteristic of harmonic torsional vibrations of single crankshaft and that of corresponding harmonic actuating torque was investigated. The results show that the phase of harmonic actuating torque greatly influences the calculate torsional vibrations, and the phase of actuating torque must be taken into account in calculation of torsional vibrations. It is feasible to use phase characteristic of harmonic torsional vibrations of crankshafts to diagnose the misfire of an individual cylinder.