Thermotropic liquid crystalline polyurethanes (LCPUs) were synthesized through the polyaddition reaction of 2,4-toluene diisocyanate (2,4-TDI), 4,4'-diphenylmethane diisocyanate (MDI), or o-toluidine diisocyanate (ODI) with 4,4'-bis(6-hydroxyhexoxy)biphenyl, and the effect of the structures of the diisocyanates on the properties of LCPUs were investigated. Intrinsic viscosities of the polymers were in the range of 0.23-0.30 dL/g. Mesomorphic behavior of the polyurethanes were investigated by differential scanning calorimetry, polarized optical microscopy, and wide-angle X-ray scattering. Different mesomorphic behaviors were observed according to the different structural characteristics of diisocyanates. Polyurethanes employing 2,4-TDI and MDI exhibited monotropic behaviors, while that with ODI showed enantiotropic behavior.