Abstract: Tests on low-cycle fatigue under uniaxial tension-compression and multiaxial nonproportional loading of two typical strain paths and the observation on micro structures of 316L and 316LN austenitic stainless steels are made. The effect of nitrogen element on the characteristics of low-cycle fatigue under uniaxial and multiaxial nonproportional loading and micro structures of stainless steels are analyzed with emphasis. The micro mechanism of the dependence of nonproportional cyclic additional hardening and low-cycle fatigue life on the nitrogen element is studied thoroughly.