Copolymers composed of 11-(4＇-cyanobiphenyloxy)undecyl acrylate (11CBA) and acrylic acid (AA), poly(11CBA-co-AA)s, were prepared by changing their molar ratio, and their structures were investigated under various temperatures. From wide- and small-angle X-ray studies, it was found that poly(11CBA) and poly(11CBA-co-AA)s with a mole fraction of 11CBA more than 0.26 showed the smectic A (SmA) phase with a layer spacing (d(2)) of 46-58 Angstrom, to which the mesogenic moiety of 11CBA attached perpendicularly with a side-by-side spacing of 4.3 Angstrom (d(1)). The ordering as well as the d(2) spacing of the smectic layer increased by incorporating an appropriate amount of AA. At the same time, the isotropic temperature of the copolymer increased with AA content. The enhanced ordering and the thermostability by introducing AA were explained in terms of the cooperative hydrogen interaction between the carboxylic groups of AA.