Although naturally mummified corpses have frequently been found in medieval tombs of Korea, there have been no scientific reports on the physical status of the mummies. In this study, we first tried to investigate the mummy using radiological methods. On physical examination, there were no findings suggestive of the cause of the death. From the radiological studies, we found that some internal organs were well preserved, while the bones showed similar appearances to those of living cases. From the stage of development of the teeth and carpal bones, the age of the mummy was estimated to be 4.5 to 6.6 years old. Although the exact cause of the mummification could not be clearly elucidated, it may be due to the lime-soil mixture outside the outer coffin, completely sealing the inner coffin from the outer spaces. In this study, as the mummy in the medieval tomb has proved to be well preserved over the last 400 years, similar cases could give invaluable clues to the physical status of medieval Koreans.