The elastic modulus of randomly oriented, glass fiber mat reinforced plastics after hydrothermal aging was estimated introducing finite element analyses and the damage mechanics. The debonding between fiber and matrix of the hydrothermal aged fiber-reinforced plastics (FRPs) was dealt with as the internal damage, and the damage mechanics were introduced to the finite element analyses. The damage angle around the fiber greatly affected the modulus reduction of the aged FRPs independent of the damage thickness. The internal damage of the aged FRPs was determined quantitatively by the weight loss due to water immersion, and the modulus of the aged FRPs was calculated by the finite element analysis introducing the damage parameter. The calculated results corresponded well to the experimental results, and it was analytically clarified that the modulus reduction of the aged FRPs was caused by the occurrence of the debonding between fiber and matrix.