A batch of turbine blade castings were produced by using a third generation single crystal superalloy. After solution heat treatment, a large number of bright white spots with irregular shapes were observed on the blade surface. Through metallographic detection of the blade sections, a distinct surface layer was exhibited, in spite of the uniform γ phase structure in the internal area of the castings. The bright white substrate of the surface layer is determined to be continuous γ’ phase composed of Ni, Al and Ta elements. In the surface layer, a lot of lamellar TCP phases precipitated from the γ’ phase matrix. The element distribution near the surface of the casting was measured and analyzed. It was confirmed that the surface layer of the casting was a typical Cr-poor layer, leading to the phase transition from γ to γ’ and the coupled growth of γ’ and TCP phases. The depletion of Cr in the surface layer also resulted in the residual of γ/γ’ eutectic between the surface layer and the internal area.