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The present study was aimed at determining the surface strain/stress state in an Ni-based single-crystal (SC) superalloy that was subjected to two different cooling rates from solid solution temperature through using the X-ray diffraction (XRD) method. The normal stresses σ 11 s and σ 22 s were determined, then the Von Mises stresses ( σ V M s ) were derived from them. Field emission gun scanning electron microscope (FEG-SEM) and transmission electron microscope (TEM) micrographs were taken to illustrate the strain/stress state change. The precipitation of the secondary γ′ phases in the γ phase and the formation of the dislocation in the interphase upon a slower cooling rate caused the γ phase lattice distortion to increase, so a larger σ V M s of the γ phase was realized in comparison to the faster cooling sample. For both of the two cooling modes, we found that the σ V M s of the γ′ phase increased due to the growth of the γ′ phase during the aging process. Also, the aging process led to pronouncedly anisotropic lattice mismatches in the {331} and {004} planes. In addition, the surface strain/stress states of a cylinder sample and a tetragonal sample were also studied using a faster cooling rate, and σ 11 s and σ 22 s were analyzed to explain the influence of the shape factor on the stress anisotropy in the [001] and [ 1 1 ¯ 0 ] orientations. The strain in the [001] orientation of the γ phase is more sensitive to the shape change.