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We report the application of a novel flat-cladding optical fiber Bragg grating sensor array in the characterization of low cycle fatigue deformation behavior of an extruded AZ31 magnesium alloy. The results obtained from optical fiber sensors showed strong asymmetry of stress-strain hysteresis loops, especially at high strain amplitudes, due to the occurrence of twinning in compression and detwinning in tension. At a given total strain amplitude, the degree of asymmetry of the hysteresis loops decreased with increasing number of cycles. The alloy also exhibited strong cyclic hardening at higher total strain amplitudes. All of these observations were in good agreement with those measured from an extensometer, and with those reported in the literature. In particular, the plastic strain amplitudes evaluated from optical fiber sensors demonstrated a nearly perfect agreement with those obtained from the extensometer, further corroborating the suitable use of the optical fiber sensors as an important tool in the evaluation of cyclic defromation behavior of materials.