Selected mechanical properties of symmetrically (±45°) reinforced 16‐lamina graphite/epoxy composites have been found to be affected by sub‐Tgannealing. Postcured specimens of Thornel 300 graphite/Narmco 5208 epoxy were sub‐Tgannealed at 140 °C for ca. 10, 102, 103, 104, and 105min, with a prior quenching from aboveTg. The ultimate tensile strength strain‐to‐break, and static toughness of the composite materials were found to decrease as a function of sub‐Tgannealing time. No weight loss was observed during the sub‐Tgannealing. The time‐dependent change in properties can be explained on the basis of physical aging which is related to free volume decrease in the non‐equilibrium glassy state of network epoxies. The results imply possible changes in composite properties with service time.