Molecular alignments of three types of liquid crystalline materials on polar solid surfaces have been investigated by the use of polarized optical microscopy. On an amino-coated glass substrate, p'-alkyl-p-eyanobiphenyls, having one polar cyano group as a terminal moiety, exhibits homeotropic alignment; that is, the molecules align perpendicular to the substrate, whereas p'-pentyloxy-p-cyanobiphenyl and p-(pentylbenzoic acid)-p-cyanophenyl ester, having two polar groups as a terminal and a spacer, appear to align parallel to the substrate surface. These results indicate that the position and the number of polar groups in a liquid crystalline molecule play an important role in the alignment. On a poly(vinyl alcohol)-covered substrate, which has polar hydroxyl groups, different alignments are observed. It has also been found that the alignments depend on the atmospheric humidity.