The production flow sheet for test steel is 2 t medium frequency induction furnace-200 kg ESR (Φ180 mm) -annealing-upset forging to Φ325 mm-spheroidal annealing-quenching at 1050 ℃-tempering at 590 ℃，furnace cooling. The effect of 0 - 0.001 0% Mg on structure and mechanical properties of hot-working-die steel H13 (/% : 0.40-0.41C

0.98-1.01Si

0.29Mn

4.95-5.08Cr

1.21-1.22Mo

0.91-0.93V

0.023-0.027P

0.006-0.007S) has been tested and studied. Results show that with increasing Mg content in steel from 0 to 0. 0010%

the average tensile strength and HRC value of test die steel H13 increases respectively from 1617.6 MPa and 45.7 to 1702.9 MPa and 47.8 ； Mg is available to increase the stability of tempered martensite in steel

in high temperature tempered die steel H13 containing Mg

a lot of tempering troostite occurs in structure and partial globular carbides with size less than 10 μm precipitate around Mg-Al inclusions

it is favourable to improvement of performance of steel.