Abstract: Ran Menglong, Liu Dehua, Zhang Jingqiu, Tu Ping, Yang Shuxia, Li Hang Department of Dermatology and Venereology, Perking University First Hospital, Beijing 100034, China Corresponding author: Li Hang, Email: drlihang@126.com 【Abstract】 Objective To investigate imaging characteristics of human skin in different sites by using 20?MHz and 50?MHz high?frequency ultrasonography, and to compare the reliability of skin thickness measurement by the above two approaches of ultrasonography. Methods A total of 39 healthy volunteers aged 18 - 39 years were enrolled into this study. Then, 20?MHz and 50?MHz ultrasonography were separately performed to image 20 different sites on the body, and the thickness of the epidermis and dermis were measured and evaluated by 3 dermatologists independently. The ultrasonic images were analyzed, and the intraclass correlation coefficient （ICC） was used to assess and compare the reliability of skin thickness measurement by 20?MHz and 50?MHz ultrasound. Results The ICC values were less than or close to 0.7 in epidermal thickness measurement at almost all the tested sites between 20?MHz and 50?MHz ultrasono?graphy, suggesting poor reliability. However, 20?MHz and 50?MHz ultrasonography showed excellent reliability in dermal thickness measurement with the ICC greater than 0.75 at almost all the tested smooth and flat body sites. The 20?MHz ultrasound could provide clear images of the dermis and subcutaneous tissue, while 50?MHz ultrasound could only provide clearer images of the dermis in details. In some uneven sites with complex anatomical structures, 20?MHz and 50?MHz ultrasound probes had their own advantages and disadvantages in imaging depth and detail resolution. Conclusions The 20?MHz and 50?MHz ultrasono?graphy both can serve as non?invasive imaging techniques to show structures of the epidermis and dermis better. In different parts of human body, ultrasound frequencies should be selected according to their imaging characteristics.