Reliability is very important for the further development, commercialization and miniaturization of microelectromechanical systems (MEMS). In particular, concern arises about time-dependent degradation such as fatigue for MEMS with flexural elements because they are used in cyclic loading. This study investigated the time-dependent degradation of silicon micro-resonating structures. The test structure, designed and fabricated by micromachining, consisted of suspended beams, shuttle, combs and electrodes. It was operated at resonance mode by applying AC voltage with a function generator and the change of resonant frequency was detected. The failure of a notched beam was detected by the saturation of the decrease in resonant frequency. The test structure showed a decrease in resonant frequency with cycles that was attributed to stiffness degradation due to fatigue crack growth at the notch tip. By analyzing the test structure as a spring-mass system, the variation of stiffness of a notched beam with cycles was obtained from the resonant frequency. From this relation and the stiffness-crack relation, crack growth with cycles was calculated. Finally, the lifetime of the test structure was calculated and compared with experimental results.