The evolution of the raft structure in the Ni-based single-crystal superalloy TMS-138 crept at 1100 °C and 137 MPa has been investigated quantitatively by the Fourier analysis of scanning electron microscope (SEM) images. This method of analysis quantifies the extent of tilting of the γ/γ′ interface during creep deformation. The analysis is carried out on two different crystallographic planes, (100) and (110), on which SEM observations of deformation microstructures are made, to see if there is any difference in the results of analysis depending on observation planes. The tilt angle of the γ/γ′ interface, which is deduced by the Fourier analysis, increases with the accumulation of creep strain. The rate of the increase in the tilt angle is significantly large in the tertiary creep stage. The result of analysis does not depend significantly on crystallographic planes on which SEM observations of microstructures are made.