Rocking-Mass Gyroscope (RMG) is a dual-axial symmetry vibrating mass gyroscope, whose operational modes are completely uniform, and values of frequency are equal. RMG has the potential to be the gyroscope with high sensitivity. Predicting the frequencies for the operational modes of RMG is critical. The natural frequency analytical formula of RMG is developed by using the assumed modal method and vector composition method. The FEM simulations and experiments validate the analytical formula. The sensitivities of natural frequency to different parameters are also analyzed. The presented results provide references for optimization design of RMG.