Monopolar radiofrequency (mRF) devices have been shown to be clinically effective for treating aging skin, but there are few histologic studies about the mechanisms. To histologically analyze chronologic and quantitative change in collagens after mRF treatment to determine the mechanisms of the antiaging effect. Five patients were enrolled in this study. Skin specimens were taken before and 1 and 3 months after treatment. Immunostaining was performed to determine change in type I and III collagen levels and stem and other cell counts in skin layers. In all cases, both types of collagen significantly increased after irradiation in the dermis (p < .05), and their changes were noticed uniformly in all layers. No significant change was noticed in stem and other cell counts. This study histologically demonstrated that type I and III collagen increased significantly in the dermis after mRF treatment. The amount of stem cells did not affect the increase in collagens.