The correlation between the optical transmission loss and the weight loss of thermally deteriorated epoxy resins was analyzed using near infrared (NIR) spectroscopy. Using three types of acid anhydride cured epoxy resins, the optical transmission losses at 850 nm of each resin and their weight losses were measured during the accelerated heating tests. At longer heating times the color of each resin darkens and its weight decreases, i.e., the values of the optical transmission loss and the weight loss are both increasing gradually. Both of tendencies are similar in magnitude, and the optical transmission loss of each resin has a good correlation with its weight loss. It is postulated that the cross-linking was increased in the resin by the thermal oxidation reaction, and that the simultaneous elimination reaction occurs from the surface. As the thermally oxidized structure is more conjugated, the optical transmission loss is increased in the shorter wavelength region, and thus theoretically, one can predict the weight loss of the thermally deteriorated epoxy resins non-destructively by measuring the optical transmission loss using NIR.