Recently, the awareness of hair damage has been increasing, but the ultrastructual damage mechanism has not been thoroughly analyzed. We determined that we could assess the degree of hair damage through analyzing the parts of the ultrastructure which were subject to damage by visualizing water distribution in a damaged hair. The damaged hair which was swelled with water was frozen and fixed with liquid nitrogen, then a fine cross section of the hair obtained with a microtome, was observed with a cryo scanning electron microscope (cryo-SEM). Due to sublimation of water in cryo-SEM, the region containing much water tends to be observed as a cavity. As a result, many cavities, which were thought to be the cortex CMC (cell membrane complex), intermacrofibrillar matrix and melanin granules, were observed only in the damaged hair, not in the undamaged hair. Therefore, such ultarastructures were thought to be more susceptible to damage, resulting in water-localization to these structures. In addition, the extent of cavities in the cryo-SEM image was correlated with the physical properties of water swelling and rupture strength due to tension. The degree of hair damage can be analyzed by observing cavities in the hair with cryo-SEM.