Abstract: By using OM、SEM、EDS、impact and tensile testing machine, microstructure and mechanical properties of large size of H13 steel forging piece were studied. The results were that because of "size effect",H13 steel of large size had severer carbon segregation,even with massive white ferrite massive region existed;eutectic carbide was V-riched MC type carbide,and at 1/2R position,the size of eutectic carbide was largest,which was likely related with solidification characteristics of large size electroslag ingot;impact toughness at center area was low and the fracture surfaces were tear ridge and quasi cleavage fracture surface,belonging to brittle fracture,plasticity increased from center to edge area and at 1/2R area,because of large size of eutectic carbide,plasticity had trough value.