To further study the sub-T-g heat flow transition of a cured epoxy resin, cured samples with different thermal history were investigated using torsion pendulum ananylsis (TPA) and thermal mechanical analysis (TMA). The results indicate that sub-T-g heat flow transition could be related to the molecualr relaxation from 20 degreesC to the alpha -peak, and that frozen-in extra free volume is necessary for the appearance of sub-T-g heat flow transiton.