Abstract: The multi-degree of freedom model has been established to describe the dynamic behavior of a planet gear train of the 2K-H type with five planet gears. The numerical integration of the equations of motion for the system has been done by the use of Gill's method. The dynamic response of the system under the stiffness excitation induced by the mesh between gears has been obtained,and the dynamic load of gears has been determined. The effects of the contact ratio of gears and the floating motion of the centroid of the sun gear on the dynamic characteristics have been discussed emphatically during the mathematical manipulation.