This paper is concerned with the coupled lateral-torsional vibration of rotors trained by gears. First, numerical calculation technique using transfer matrix method is presented to obtain both free and forced vibrations, where, for the simplicity of analysis, tooth rigidity is assumed to be a linear spring, and backlash and tooth profile error are neglected. Then the numerical calculations were carried out in three cases; (1) the force acting on the contact line is only a function of rotations at gears, (2) the force acting on the contact line is a function of both the rotations and the flexure at gears, and (3) two rotors are not coupled. The eigenvalues, eigenmodes, and the responses of unbalanced mass are calculated and the results are discussed.