The expansion curves of phase transformation in hot work die steel H13- 0.34C
0.39Mn
0.91Si
5.11Cr
1.34Mo
0.91V
0.0015S
0.009P with austenitizing temperature 1060 ℃ at different cooling velocity between 20 ℃/s and 100 ℃/h were determined by Gleeble 1500D thermal simulation test machine and the continuous cooling trans- formation(CCT)diagram has been obtained using Origin software by expansion principle.The results obtained that the martensite structure in H13 steel was got as the steel austenitized at 1060 ℃ and cooled at the cooling velocity between 20 ℃/s and 15 ℃/min.