In order to obtain the improved strength and toughness for high-pressure low-pressure rotor shaft forging, fundamental studies using laboratory heats were performed on the 9CrMoV base materials, and effects of chemistry on toughness and creep rupture strength were investigated. From the investigation, it is showed that the superclean 9CrMoVNiNbN steel with reduced Si and Mn contents and Ni addition provides a superior strength versus toughness balance. Based on these fundamental studies, a trial high-pressure low-pressure rotor shaft forging with diameter of low-pressure section of 1750 mm and diameter of high-pressure section of 1200 mm was successfully manufactured from the diameter of 1800 mm, and the weight of 65 ton ESR ingot. From the evaluation test results of this trial rotor forging, homogeneous distribution of chemistry was confirmed and low impurity contents was observed in the whole forging. The superior strength and toughness were confirmed with good creep rupture strength. The FATT at the center of low-pressure section was −3‡C with the tensile strength level of 870 MPa. From the results of fracture toughness test, low cycle fatigue test, and isothermal aging test, superior mechanical properties were demonstrated. Thus, the superclean 9CrMoVNiNbN steel with reduced Si and Mn contents and Ni addition, is particularly suitable to the high-pressure low-pressure rotor material for advanced combined cycle power plants.