A parametric finite element model for planetary ring gear is presented to investigate the effects of boundary conditions on the ring vibration characteristics. The modal analysis indicates that there exists five categories of vibration modes, i.e. rigid-body motion, in-plane bending, out-plane bending, torsion and expansion. The effects of internal and external boundary conditions such as constraint stiffness and positions on the ring vibration characteristics are thoroughly explored. The numerical simulations show that the in-plane bending modes are more sensitive to the boundary conditions when compared to other vibration modes. Furthermore, the comprehensive effects of internal and external boundary conditions on ring gear vibration are very complicated. Nevertheless, an optimal combination may be adopted to achieve a high dynamics performance for the ring gear.