Thermoelastic response of a cross-linked main-chain polyester was examined in the nematic temperature region from 80 degrees C to 150 degrees C. The monodomain nematic elostomer decreased its length in the orientation direction by 45% on heating from 80 degrees C to 150 degrees C and recovered it on cooling to 80 degrees C without change in the degree of orientational ordering. This reversible distortion of the nematic elastomer is well explained by the configurational change of the polymer chain including hairpin folding, the probability of which is proportional to the Boltzmann factor of hairpin energy.