Abstract: Objective To study the cell killing effect on isolated sarcoma 180 cells by ultrasound activating Protoporphyrin IX and to explore its potential biological mechanism .Methods The sonodynamical effect was investigated on S180 tumor cells exposed to the combination of 120μmol/L protoporphyrin IX (PPIX) and focused ultrasound at the frequency of 2.2MHZ and an intensity of 3W/cm2. The livability of cells was evaluated by trypan blue staining method. Scanning electron microscope (SEM) observation of the surface of cells was performed to evaluate the morphological changes induced by ultrasonic irradiation. The generation of oxygen free radicals in cell suspensions was immediately detected after treatment by the active oxygen detection kit. Oxidative stress was assessed by measuring the activities of key antioxidant enzymes (ie, Superoxide dismutase[SOD], Glutathione peroxidase [GSH-PX], Catalase [CAT]) in S180 cells after SDT. Results The cell damage rate of ultrasound combined with PPIX was significantly higher than treatment with ultrasound alone only, and PPIX alone had no killing effect on S180 cells. Enzymatic chemical methods showed the content of MDA significantly increased after treatment, while the activities of key antioxidant enzymes in tumor cells all decreased at different levels, and which were associated to the generation of oxygen free radicals in cell suspension after treatment. Conclusion Oxygen free radical may play an important role in improving the membrane lipid peroxidation, decreasing the activities of key antioxidant enzymes in cells, and the biological mechanism might be involved in mediating the killing effect of S180 cells in SDT.