The effects of alloying elements on the hardenability, toughness and the resistance to stress corrosion cracking have been evaluated in the 1 to 3 mass%Cr low alloy steels for 1600 mmφ class large-sized turbine rotor forgings for geothermal power generation. The chemical composition suitable for the rotor forging which satisfied the required material properties and which reduced the C-segregation in the center portion of the ingot was chosen under the restrictive condition on the upper limit of Ni and Cr contents suitable for turbine rotors in a geothermal corrosive environment. An 8 ton sand mold ingot of the selected chemical composition was manufactured and the reduction of the C-segregation was verified. It is concluded that the selected 2.25Cr-0.9Ni-0.6Mo-0.25V-0.15Si-1.0Mn-0.25C steel (mass%) is hopeful as the chemical composition for large-sized turbine rotor forgings for geothermal power generation.