Excess enthalpy was measured during thermal aging for poly(6-(4'-cyanobiphenyl-4-yloxy) hexylacrylate) at various temperatures. The relaxation behavior was analyzed on the basis of the activation-energy spectrum (AES) model. The relaxation time was evaluated and its dependence on the aging temperature was explained using an Arrhenius-type equation. The apparent activation enthalpy was 220kJ/mol, lower than those values reported for other polymers. The AES obtained showed a maximum value. The measurement was also carried out with different molecular weights, and the systematic variations could be observed for the AES.