The anchoring structures of binary mixtures of 4’-n-octyl-4-cyanobiphenyl (8CB) and 4’-n-dodecyl-4-cyanobiphenyl (12CB) on molybdenum disulfide (MoS2) are directly observed by scanning tunneling microscopy (STM) in order to investigate the boundary condition of liquid crystal mixtures at the molecule substrate interface. On the basis of the highly reproducible STM images, the anchoring structures of binary mixtures are clearly divided into three categories : a homogeneous single-row type consisting of only 8CB (in the case of 8CB:12CB = 90:10, 80:20, and 70:30 (mol %)), an inhomogeneous (mixed) double-row type consisting of 8CB and 12CB (60:40, 50:50, 40:60, and 30:70), and a homogeneous double-row type consisting of only 12CB (20:80 and 10:90). The ratio of 8CB and 12CB adsorbed on MoS2 substrate in all mixtures is different from that in the bulk. Furthermore, a new phase transition only available at the boundary is confirmed for the first time. The correlation between those anchoring structures of liquid crystal mixtures and the bulk phase diagram is discussed from the viewpoint of anchoring phase formation and selective adsorption of mixtures at a molecular level.