Abstract: The laboratory experiments of inclusions in H13 die steel modified by Ca treatment were employed in a vacuum induction furnace and the inclusions were analyzed using a scanning electron microscope equipped with an energy dispersive spectrometer in the present study.The evolution of inclusions in H13 steel was analyzed based on thermodynamic calculation.And the component and structure of inclusions in H13 steel after Ca treatment were investigated.The results show that the particle size is modified to tiny and the roundness increases after calcium treatment, the ratio of the sum of cross sectional area of all the inclusions to the total area of the observed view fields decreases.The inclusions change from Si O₂-Al₂O₃to Ca O-Si O₂-Al₂O₃- (MgO) , and all the inclusions are in the1400℃low melting temperature region of the Ca O-Si O₂-Al₂O₃- (MgO) ternary phase diagram at the calcium mass content of 12×10^-6.Ca S is unable to form after Ca treatment due to the high T.O content of current steel.The Al₂O₃in complex inclusions would be totally eliminated as the mass ratio of Ca/T.O reached 0.3.The (Mn, Cr) S precipitated on oxide inclusions is detected, and the morphology of complex inclusions is depended on the degree of segregation of Mn, Cr and S.