On ECRS-6 , we have presented first results of the researches of fatigue degradation and martensitic transformation of austenitic stainless steel AISI 321 by neutron diffraction stress analysis. A series of samples preliminary ex-situ cyclically fatigued at the frequency of 5 and 0.5 Hz was in-situ tested on the stress rig of the ENGIN instrument. In the high cycle fatigued (HCF) samples, the applied stress-elastic strain responses of austenite and martensite phases were find out to be strongly different as compared to the low cycle fatigued (LCF) samples, in which they are close. Moreover, the martensite Poisson ratio in the HCF-samples is almost twice to that of observed 0.28-0.30 in austenite and in both phases of the LCF-samples. With the purpose to search the reason of such unusual behavior of the martensite phase, one of the HCF-samples has been anew in-situ tested on the stress rig of the ENGIN-X in: 1) a LCF-mode at the frequency of 0.1 Hz to increase the fatigue level, and 2) a quasistatic mode to measure the applied stress-elastic strain responses of both phases. Also, two of the LCF-samples have been subjected to the ex-situ secondary HCF-testing at the frequency of 5 Hz and again in-situ measured on the ENGIN-X stress rig. Results of the mechanical characterization of phases in the twice fatigued austenitic stainless steel are presented and discussed.