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Thermomechanical fatigue (TMF) and isothermal fatigue (IF) behaviors of niobium-added ferritic stainless steel were presented in the temperature range from 100 °C to 800 °C; further, we proposed a new fatigue failure criterion to predict the lifetimes of the components for different thermal cycle ranges in the TMF condition. The test results revealed that the TMF life decreased monotonously as the maximum temperature increased higher than 600 °C. By modifying the Coffin-Manson equation using the temperature factor, we obtained a new parameter that was successfully correlated with the lifetime under a different maximum temperature. The stress-strain hysteresis loops and cyclic stress responses during fatigue cycling were compared to elucidate the different fatigue behaviors under TMF and IF conditions.