In recent years, surface γ/γ′-eutectics have been observed to form continuous layers along the peripheries of the outer surfaces of Ni-based single crystal (SC) castings. This type of solidification defect poses specific concerns for turbine blade production because large surface eutectics are very difficult to completely dissolve during practical heat treatment. In the present work, a third generation SC superalloy was used to cast turbine blades and cylindrical bars. However, no eutectic layer was observed on the outer surface of the blade aerofoils and bars in hundreds of castings. Instead of the lateral eutectic enrichment on the outer side surface, a non-uniform distribution of eutectics along the solidification length was detected. Eutectics were found to accumulate on the upper surface of solidification units such as platforms whereas less eutectics were observed on the bottom surface. Especially on the inclined upper surface at the junctions between platform and airfoil, a large amount of eutectics were accumulated so that only a small part could dissolve during conventional solution heat treatment. This phenomenon, called the eutectic accumulation on the upper surface, could be attributed to the upward solutal flow of eutectics forming elements through diffusion and convection, which is finally stopped beneath the upper surface. The factors influencing the upward eutectic accumulation are the chemical composition of the alloys, the process conditions, and the local geometry of the castings, such as the platform thickness and the inclination of the upper surface. Another surface effect was also observed on the eutectic dissolution during heat treatment process. It was found that the eutectics in the outer surface area were dissolved completely during heat treatment process than that in the internal one. The possible explanation is that at high temperatures, the eutectic forming elements Al and Ti evaporate from the casting surface faster than the other elements, as experimentally confirmed by chemical measurement.