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The effect of plastic anisotropy, which is caused by the arrangement of slip systems, on the creep strength of notched specimens of Ni-based superalloy single crystal, CMSX4, is described. It was revealed that the creep strength of the notched specimens was affected by the crystallographic orientation, not only in the tensile direction, but also in the thickness direction. The creep strength was superior in the notched specimen with the [011] tensile and [011] plate-normal orientations, whereas the notched specimen with [011] tensile and [100] plate-normal orientations exhibited extremely poor creep strength. In the case of specimens whose tensile orientation was [011], results of the creep-rupture tests at 750 °C were in agreement with the assumption of the operation of {111} slip systems. The creep strength in the notched specimen with [001] tensile and [100] plate-normal orientations was superior to that in the notched specimen with [001] tensile and [011] plate-normal orientations. When the tensile orientation was [001], the results of the creep-rupture tests were in agreement with the assumption of the operation of {111} slip systems during primary creep region and {111} slip systems during secondary creep region.