High manganese steel was maintained stability of Non-Magnetics performance. Fatigue tests were carried out under constant stress amplitude, using a non-magnetic high manganese steel. The fatigue crack growth mechanism of the high manganese steel was clarified from results such as observation of crack growth path and fracture surface. The result of getting this study was shown as following: 1) Remarkably ${Delta}Kth$ of the high manganese steel is big with about 3 times of the general steel product. 2) In the low ${Delta}K$ value region, da/dN is dependent on Kmax, and in the high ${Delta}K$ value region, it is dependent on ${Delta}Keff$. The reason of this behavior is crack closure due to fracture surface roughness and fretting oxide. 3) It seems to ease the stress concentration of crack tip crack growth behavior in the ${Delta}Kth$ vicinity by the generation of the secondary crack.