Abstract: The low cycle fatigue (LCF) behaviors of Z3CN20- 09M austenitic stainless steel were tested by the method of radial strain control at room temperature and 350℃. The steel presents cyclic hardening followed by cyclic softening, and the degree of cyclic hardening depends on temperature and strain amplitude. With the increase of strain amplitude, the LCF life of the steel decreases, but the stress amplitude for the same cycles increases. Temperature has great effect on the LCF behaviors of the steel, the degree of cyclic hardening at 350℃ is higher than that at room temperature, and the LCF life at 350℃ is also higher than that at room temperature for the same strain amplitude. Through in-situ observations in fatigue testing, slip planes within austenite, inclusions, and austenite/ferrite phase boundaries are considered to be the possible nucleation sites of fatigue cracks. The incongruous deformation abilities of austenite and ferrite cause stress concentration in the phase boundaries and become the preferential propagating paths of fatigue cracks.