Nematic-isotropic transition temperature was determined for 4-(4-X-phenyliminomethyl)phenyl 4-Y-benzoates and 4-X- phenyl 4-(4-Y-phenyliminomethyl)benzoates, where X and Y are CN, CH3O. NO2, Cl, Br, N(CH3)(2), CH3, F, or CF3 groups. The transition temperature in the absence of the dipole-dipole interaction was estimated by extending Van der Veen's approximation. While a large positive deviation is found for the CH3O-NO2 and CH3-NO2 derivatives, indicating the significant contribution of dipole-dipole interaction to the nematic thermal stability, no deviation in the CH3O-CN derivatives suggests the anti-parallel molecular association in the mesophase.