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In the present experiments, the casting samples of a third generation single crystal superalloy were solution heat treated in different atmospheres, in order to investigate the microstructure near the sample surface and to develop a suitable measure avoiding the Cr-depletion layer. Under high vacuum atmosphere, a depletion layer of alloying elements such as Cr, Co and Re was observed beneath the sample surface, with about 70 μm in depth. When the furnace chamber was filled with argon gas to protect the sample, unexpectedly, the oxidation of the sample surface occurred and the depletion incipient melting zone in a depth of about 1 mm was detected. This unusual phenomena may be attributed to the filling of the protective gas which caused the strong convection in the furnace chamber under very high temperature, in spite of the high purity of the used argon gas. When the sample was enclosed in a ceramic tube during heat treatment, the sample was located in a narrow space and isolated from the strong convection in the large furnace chamber. In this case, the reaction of casting sample with the furnace atmosphere was effectively suppressed during heat treatment process. Moreover, the evaporation of the alloying elements became quickly saturated within the narrow closed space. As a result, the serious depletion of the alloying elements from the casting surface was successfully avoided.