Enthalpy recovery measurement was used to study the physical aging behavior of a polystyrene. The aging experiments at temperatures (T-A) near T-g were conducted with aging times (t(A)) ranging from several minutes to several days. The recovered enthalpy, Delta H(t(A), T-A), increased with t(A), and then a plateau value of Delta H-plate appeared in the long time aging experiment. The master curve was successfully constructed concerning the normalized relaxation isotherm of phi for the temperature range of T-g-16