Abstract: The effect of photosensitive therapy on the blood circulation of S180 solid tumor was studied, using radioisotope technique. The tesults showed that 12 hrs after photodynamic therapy.the tumor blood circulation was affected seriously. This photosensitive impairment was most severe 24 hr after the treatment. 48 hr later, the tumor blood circulation got improved, indicating that repair processes of tumor blood vessels were operative. The study further confirmed that destruction of tumor blood vessels seems to be one of the important mechanisms of photosensitive therapy in the treatment of cancer.