In this study, a dynamic model of a parallel-axis helical gear system is developed. A linear time-invariant dynamic model of a gear system is proposed. This model is constructed by using a three-dimensional model of a helical gear pair as the beam element in combination with a finite element shaft structure. Bearing flexibility is included in the model as a linear spring, and the torsional, transverse (line of action), rotational, and axial vibratory motions of the gear pair are also considered. For the gear pair model, the static transmission error is regarded as the excitation of the gear mesh, under the assumption that the gear mesh stiffness is a constant value. To validate the proposed model in predicting gear vibratory motions, the experimental results of the helical gear system are presented and compared with the predicted results. The predicted results are found to agree well with the experimental results in terms of natural frequencies and forced vibrations.