Effects of various amount addition of heavy rare earth yttrium on microstructure and mechanical properties of high manganese steel was studied. Results showed that with the addition of rare earth increased

austenite grain size refined at first and then enlarged. Tensile strength and impact toughness improved firstly and then decreased . The size of austenite reached the minimal value （109.88μm） while tensile strength and impact toughness reached the maximum value（200.64 MPa and 1 088.11 KJ/m2 respectively）when yttrium content was optimal and found to be 0.3wt.% （remains 0.023 wt.%）.Metamorphic mechanism research indicated that adding small amount of Y formed less rare earth oxide nucleuses to refine the grain size

the ensile strength and impact toughness could not improve obviously. Excessive amount addition poisoned the nucleuses causing the grain size growth

tensile strength and impact toughness were destroyed because of the second phase continuously distributed nearby the crystal boundary.