Isothermal physical aging of a glass fiber/epoxy composite was examined at different aging temperatures(T-g) and degrees of conversion (monitored by the glass transition temperature, T-g) by means of the TEA torsion pendulum technique. The range of aging temperature was from 10 to 130 degreesC; the conversion was systematically changed from T-g 76 degreesC to T-g=177 degreesC (fully crosslinked). The effect of isothermal physical aging was manifested as perturbations of the modulus and mechanical loss vs. temperature in the vicinity of T-g for all conversions. The rate of isothermal physical aging determined from the change of modulus with aging time at fixed aging temperature decreased and then increased with increasing conversion below T-a = 90 degreesC. There exists a superposition in aging rate vs. (T-g- T-g) by shifting horizontally and vertically. This implies that the physical aging process is independent of the change of chemical structure as conversion proceeds. It has been found that water absorbed at the aging temperature below 70 degreesC during isothermal physical aging lowers the apparent aging rate. It is due to the absorbed water molecules forming strong polar interactions with hydroxyl group on network chain and reducing the segmental mobility during the physical aging.