Two new mesogenic homologous series of liquid crystalline compounds containing 1,2,4-triazole and 1,3,4-thiadiazole in the molecule viz. 1,4-bis{3-[2-(4-alkoxy-benzelideneamino)-1,3,4-thiadiazole-5-ethylenethio] 1,2,4-triazole-5-yl} phenyl and 5-(4-methoxyphenyl)-3-[2-(4-n-xybenzelidenamino)-1,3,4-thiadiazole-5-methylenethio]-1,2,4-triazole were synthesized by many step procedures. Both series have been characterized by elemental analysis, FT-IR, mass spectrometry, (1)H-NMR, and (13)C-NMR. Phase transition temperature and the thermal parameters were obtained from differential scanning calorimetery (DSC). The texture observations were performed under hot-stage polarizing optical microscopy (POM). All compounds of the first series exhibited an enantiotropic nematic mesophase. The first compound of the second series exhibited an enantiotropic nematic mesophase, and that with propyloxy terminal substituent shows one enantiotropic nematic phase on heating and a monotropic smectic A (SmA) phase on cooling, while the last two homologues exhibit a monotropic SmA phase. In contrast, the compound with an ethoxy terminal substituent does not show any liquid crystalline properties. The mesomorphic behavior has been analyzed in terms of structural property relationship.