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Recently, induction heating treatments have been applied to high alloy tool steels, for example JIS-SKH51. In rapid and short time heating, however, the influence of heating time and temperature on carbide dissolution and hardness is not fully clarified. The purpose of this study is to make clear the influence of these two factors on carbide dissolution and quenching-tempering hardness behavior of JIS-SKH51. Results obtained are as follows; 1) The initial stage of the holding at hardening temperature is characterized by the dissolution of small M 6 C carbides. 2) Increase of carbon and alloy elements in matrix before induction heating contributes to the increase in hardness in short time heating. 3) Finally, we applied induction hardening to two sizes of SKH51 forming rolls using the fundamental knowledge obtained in this study. Surface hardness higher than 65HRC and satisfactory hardened depth for each size of rolls were obtained..