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This paper describes fatigue damage evaluation for micro-nanoscale single-crystal silicon (SCS) structures toward the reliable design of microelectromechanical systems subjected to fluctuating stresses. The fatigue tests, by using atomic force microscope (AFM), nanoindentation tester, and specially developed uniaxial tensile tester, have been conducted under tensile and bending deformation modes for investigating the effects of specimen size, frequency, temperature, and deformation mode on the fatigue life of SCS specimens. Regardless of frequency and temperature, the fatigue life has correlated with specimen size. For example, nanoscale SCS specimens with 200 nm in width and 255 nm in thickness have showed a larger number of cycles to failure, by a factor of 105, at the same stress level, as compared to microscale specimens with 48 ??m in width and 19 ??m in thickness. Deformation mode has also affected the lifetime; however, no frequency and temperature dependences have been observed unambiguously in the S-N curves. The stress ratio parameter corresponding to the ratio of peak stress to average fracture strength has enabled us to estimate the lifetime for each deformation mode. To predict the fatigue life of SCS structures regardless of deformation mode and specimen size, we have proposed an empirical parameter that includes the resolved shear stress. The mechanism of fatigue failure of SCS structures is discussed from the viewpoint of dislocation slip, crack nucleation, growth, and failure through observations using AFM and scanning electron microscope.