M23C6 carbide is a common precipitate phase present in Ni-based superalloys. The carbide is generally an alloy carbine and often contains more than one metallic element. In this study, the site preference of metallic elements in M23C6 carbide in a Ni-based single crystal was determined by means of aberration-corrected High-Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF-STEM) technique and quantitative STEM (QSTEM)-based image simulation method. The carbide was found to contain mostly Cr and Re, with a small amount of Mo. The HAADF-STEM and QSTEM-based simulation analyses indicate that heavy atoms prefer to substitute Cr at 8c sites.