A stretching test of the hydrogel with a smectic A (SmA) type of liquid crystalline structure, poly(11-cyanobiphenyloxyundecyl acrylate-co-acrylic acid), poly(11CBA-co-AA), was carried out in terms of various water contents. It was found that Young's modulus of the hydrogel substantially decreased with increasing the swelling degree, q, but suddenly jumped up at q = 1.13. Wide- and small-angle X-ray diffraction studies revealed that the hydrogel underwent a mesophase transition from the SmA to smectic I (SmI) phase between these q values. When the samples were further stretched, the backward transition from the SmI to SmA phase occurred accompanied by "necking". The possible mechanism of the tension-induced mesophase transition is proposed.