In order to elucidate the antitumor effect and mechanism of action of photodynamic therapy (PDT) using the photosensitizing agent mono-L-aspartyl chlorin e6 (NPe6) and a semiconductor laser, we conducted a morphologic study on uterine cervical cancer cell lines. First, tumor shrinkage was confirmed in a tumor growth inhibition test. Next, morphologic changes after PDT were examined, and since the major change appeared to be tumor necrosis secondary to obstruction of the blood vessels around the tumor, an NPe6 cell uptake experiment was performed. The results confirmed that a significantly greater amount of NPe6 was incorporated by human umbilical vein endothelial cells (HUV-EC1) and the cervical cancer cell lines than by human umbilical cord-derived fibroblasts. Based on these findings it was concluded that NPe6 possesses tumor affinity, and necrosis secondary to vascular obstruction was postulated to be the principal mechanism of the antitumor effect of PDT using NPe6.