Poly(3,3’,5,5’-tetraethyldiphenylmethane-1,18-octadecanedicarboxamide) (poly(TEDPM-ODDA)) and poly(diphenylmethane-1,18-octadecanedicarboxamide) (poly(DPM-ODDA)) are newly synthesized and found mesomorphic phase between 493 and 502 K for the former and no mesomorphic phase for the latter by use of DSC measurements and polarizing microscope observation. In the IR spectra of enantiotropic mesomorphic poly(TEDPM-ODDA), the stepwise high frequency shift of the hydrogen-bonded N-H stretching vibration in crystalline-mesomorphic and mesomorphic-isotropic liquid phase transition suggest formation of the mesomorphic state due to balancing interchain hydrogen bonding and other interchain forces. On the contrary, non-mesomorphic poly(DPM-ODDA) the N-H bands shifts to higher frequency at melting to isotropic liquid at a stretch.