Prostheses need to show long-term functioning and stability, but, with existing test methods, it is difficult to non-destructively assess the internal structure of hard resin facing crowns. In this study, we employed the characteristics of dental optical coherence tomography scanners to evaluate the internal structure of hard resin facing crowns fitted in patients, investigate whether or not there were internal defects, and determine their rate of occurrence. We evaluated the failure of hard resin facing crowns in 37 patients（12 males, 25 females）. OCT images of 152 hard resin facing crowns from upper or lower central incisors to first premolars were obtained in order to investigate their internal defects. Internal defects（one or more voids of 200 µm）were seen in 66.4％ of all teeth in this study. The defects were located between resin layers（41.8％）, between resin and metal（35.5％）, and in the interior of the same resin（22.7％）. A reduction in the internal defect rate is strongly desired for the long-term functioning and stability of prostheses. The utility of OCT as a non-destructive test was shown in this study.

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