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We developed remaining service life diagnostic technology of phenol insulators for circuit breakers. Phenol insulators were analyzed by chemical evaluation such as ion concentration and coloration, and the results were diagnosed by the Mahalanobis-Taguchi system (MTS), which is a pattern information technology. Particularly effective items for diagnosis included ions of nitric acid, sulfuric acid, chloride, and calcium. Since there was a good relationship between the results diagnosed by MIS and the actual measurement results of surface resistivity, we found that applying MIS to the chemical evaluation results allowed deterioration diagnosis of phenol insulators instead of surface resistivity, which couldn't be measured on-site. Time was defined as the end of the service life when electrical discharge was initiated and remaining service life was predicted. The linear relationship (master curve of service life) was clarified between the real number value of elapsed time and the logarithmic value of surface resistivity at 50% RH, and remaining service life was predicted from the diagnosis results by MIS and the year in which the master curve and the threshold value intersect. Surface resistivity (threshold value) for electrical discharge initiation was derived from creepage distance, permittivity of phenol insulators, frequency, etc. We substantiated that the remaining service life of the phenol insulators could be predicted by this technology because a good relationship existed between the check examination results of the electric discharge initiation in the laboratory and the prediction results. [ABSTRACT FROM AUTHOR]