.....We observed histological and ultrastructural changes of photoaging and photorejuvenating effects of 5-aminolevulinic acid (ALA)-PDT in a UV-irradiated mice model. Methods: A total of 20 mice were divided into a control (group A) group and a UV-irradiated (photoaging) group. The photoaging group was divided according to the following interventions: photoaging only (group B), ALA application only (group C), light-emitting diode (LED) irradiation only (group D), and ALA-PDT with a light dose of 20 J/cm2 (group E). Serial skin biopsies were performed from day 2 to day 21, and transmission electron microscopic (TEM) studies were performed. Results: After UV irradiation, the amount of dermal collagen fibers decreased, and the quantity of elastotic materials increased. Following ALA-PDT application, the amount of collagen fibers increased from day 2 to day 21 and the increased elastotic materials during the photoaging period were normalized. With TEM, the decreased collagen fibers during photoaging were restored after PDT application. Also, distended dermal fibroblasts with distended endoplasmic reticulum by UV irradiation were normalized after PDT application. Conclusion: This study provides histologic evidence of the beneficial effects of ALA-PDT, even in photodamaged skin. ALA-PDT induces deposition of collagen in the dermis, normalizes elastotic materials which were induced by photoaging and may even have a direct effect on the normalization of the morphology of fibroblasts.