Hot work tool steels generally consist of the tempered martensitic structure and they are high strength materials using all strengthening mechanisms, such as strain strengthening, grain refinement strengthening, solid solution strengthening and precipitation strengthening. It is necessary to use the grain refinement strengthening that can bring both higher strength at high temperature and toughness than those of conventional steels. In this study, hot work tool steels with ultra-fine grained structure were fabricated by the mechanical alloying treatment. The powder mixture of atomized AISI H13 steel powders and Y2O3 powders was mechanically alloyed by a planetary ball mill for 360ks. The mechanically alloyed powders were packed in a stainless steel tube in a vacuum and then consolidated by hot rolling for full densification. The consolidated material was austenitized at 1293K, which was general austenitizing temperature of H13 steel, and then oil-quenched. Through this process, an ultra-fine grained H13 steel with equiaxed grains of about 0.3 microns in diameter could be obtained. A quenched hardness of the developed steel was comparable to that of an AISI M2 high speed steel produced by melting and hot-working process. Furthermore, although tempered hardness of the M2 steel decreases with increasing tempering temperature of 848K or higher, that of the developed steel does not decrease so much. Therefore the hardness of the developed steel exceeds that of the M2 steel at tempering temperatures over 923K.