We proposed an "optical loss analysis" as a novel nondestructive diagnosis for the thermal deterioration of insulating resins. Using an alkyd resin film, the optical absorption loss per unit optical path length and the glass transition temperature (T-g,T-DMA) by dynamic mechanical analysis were measured during heating tests. At longer heating times, values of the optical absorption loss and the T-g,T-DMA increased concurrently. By rescaling the heating time as a reduced time, a master curve of the deterioration for the resin was obtained. Consequently, when thermal stress is the main cause of the deterioration, one can estimate the degree of deterioration and theoretically the residual lives of resins using this optical loss analysis.