For the safe operation of high temperature equipment, it is necessary to ensure creep rupture ductility of the components from the viewpoint of notch weakening. In this study, the effect of heat treatment conditions on creep rupture ductility was evaluated and its underlying metallurgical mechanism was investigated with using a forged Ni-based superalloy Udimet520. In order to improve the creep rupture ductility without lowering the creep rupture strength, it is important to increase both intragranular strength and intergranular strength in a balanced manner. For this purpose, it was clarified that 1) secondary γ' phase within grains should be kept fine and dense, 2) grain boundaries should be sufficiently covered by M 23 C 6 carbide by increasing its phase fraction, and 3) tertiary γ' phase within grains should be redissolved before the start of creep. To obtain such a precipitate state, it is essential to appropriately select the cooling rate after solution treatment, stabilizing treatment and aging treatment conditions.