A novel nondestructive diagnosis using optical reflective analysis is proposed to evaluate thermally aged opaque insulating resins. Values of reflective absorbance (A(R)) in the visible to near infrared (IR) range were measured under a variety of aging conditions using the bisphenol-A type epoxy resin tapes containing iron oxide pigment. The A(R) values increased markedly over 180 degrees C before and after heating. This increase was due to the electronic transition absorption caused by thermal oxidation of the resin. By rescaling the aging time as a reduced time, a master curve to evaluate thermal aging of the resin was obtained. Furthermore, a similar curve was obtained using the difference (Delta A(R)) of the A(R) values between two wavelengths in the near IR range. This indicated that the surface reflectivity dependence on raggedness and dust and stain could be reduced. Consequently, when the thermal stress was the main cause of aging, the aging and residual life of the resin could be evaluated using this method.