New semi-rigid thermotropic liquid crystalline (LC) polyesters containing a twin biphenyl analogue of 1,3,4-thiadiazole in the main chain were prepared by melt polycondensation of the bismethyl ester derivative of twin 2-phenyl-1,3,4-thiadiazole having a decamethylene segment in the central part with three aliphatic diols. The polymer with an octamethylene segment forms a monotropic nematic phase and those with decamethylene and dodecamethylene segments form enantiotropic smectic phases, although their LC states are unstable. The melting and isotropization temperatures decrease with increasing length of alkylene spacers.