The nematic-isotropic transition temperature of 4-X-phenyl 4-(4-Y-benzylideneamino)benzoates, where the terminal groups X and Y are CN, CH3O, NO2, Cl, Br, N (CH3)(2), CH3, F, or CF3, was examined. The transition temperature difference between the two series, in which X or Y=CH3O and F, respectively, decreases in this order: CF3 > NO2 > Cl = Br = F > CN > CH3 > CH3O > N (CH3)(2). Thus, the groups are almost in the order of substituent constants except for the CN group, suggesting that the dipole-dipole interaction contributes significantly to the nematic thermal stability and also that the effective dipole moment of the molecule bearing CN and CH3O groups is markedly reduced in the mesophase from that of the free molecule because of the anti-parallel molecular association.