The effect of grain size on the mechanical properties of a high-manganese (Mn) austenitic steel was investigated via electron-backscattered diffraction,transmission electron microscope,X-ray diffraction,and tensile and impact tests at 25 ℃ and-196 ℃.The Hall-Petch strengthening coefficients for the yield strength of the high-Mn austenitic steels were 7.08 MPa mm0.5 at 25 ℃,which increased to 14 MPa mm0.5 at-196 ℃.The effect that the grain boundary strengthening had on improving the yield strength at-196 ℃ was better than that at 25 ℃.The impact absorbed energies and the tensile elongations were enhanced with the increased grain size at 25 ℃,while they remained nearly unchanged at -196 ℃.The unchanged impact absorbed energies and the tensile elongations were primarily attributed to the emergence of the micro-twin at-196 ℃,which promoted the cleavage fracture in the steels with large-sized grains.Refining the grain size could improve the strength of the high-Mn austenitic steels without impairing their ductility and toughness at low temperature.