The effect of grain size on the tensile properties and fatigue behavior of austenitic high Mn steel has been investigated. The recrystallized austenite grain size of the cold rolled high Mn steel was increased as the annealing temperature increased from $600^{circ}C$ to $1000^{circ}C$. Larger austenite grain size decreased the yield strength and the tensile strength, and increased the uniform elongation due to transformation of some austenite into twins or E-martensite phase during deformation. Austenite grain refinement increased the tendency to form dislocation cells, instead. The specimen annealed at $1000^{circ}C$ with large grain size showed lower fatigue crack propagation rate in low ${Delta}K$ region due to rougher fracture surface caused by formation of deformation twins during fatigue at the crack tip region.