The paper built double internal mesh planetary gear set with FEM. With one operating condition applied, the strength of sun gear, planetary gear and ring gear with HRC profile is obtained. Transmission error (TE) and gear meshing loads are calculated from further simulation. Through gear meshing stiffness formula with TE, the gear meshing stiffness between sun gear and planetary gear, planetary gear and ring gear are calculated. This FE method for gear meshing stiffness takes account of most deformations (from bearing, gear teeth load sharing, misalignment and shaft bending etc.) during gear meshing process and all these extra deformations, which are neglected in traditional gear meshing stiffness’ formula, are transformed to TE and it is a new way to calculate gear meshing stiffness with HRC profile and proved to be easy, feasible and practical.