Abstract: The modification of M2 cast high speed steel had been conducted by using complex modifier of Re, Al and N. It was manifested that network eutectic carbides were eliminated, matrix structures were refined, segregation of tungsten and molybdenum elements were relieved and the impact toughness of the steel obviously increased without hardness loss.Quenched at 1180~1200℃ and three times tempered at 560℃, the hardness of the steel maintain 65~66HRC, impact toughness increases from 8.5J to 17.0J and the modified steel had excellent fatigue resistance and wear resistance at high temperature.