Abstract: Photodynamic therapy (PDT) is a promising technique for the treatment of malignant tumors. But its photosensitizing side effects is a limiting factor for further development. In this paper, a quantitative method for measuring its side effect is described. By using this method, YHPD (a new HPD) was found to have more photosensitzeing side effect than photofrin Ⅱ and β-carotene reduced the side effect of Ji-Guang 3.This method is simple, quantitative and rapid.