Background: The sonodynamically induced antitumor effect of protoporphyrin IX (PPIX) disodium salt was studied in mice bearing sarcoma 180 solid tumors. Methods: In order to determine the optimum timing of ultrasound exposure after administration of PPIX, the PPIX concentrations in plasma, skin, muscle and tumor were estimated by measuring the fluorescence intensity of tissue extractions with a fluorescence photometer based on the standard curve. Antitumor effects were estimated by measuring tumor size and calculating the average survival time of tumor-bearing mice after sonodynamic therapy; additionally, the morphological changes of sarcoma 180 cells were evaluated by transmission electron microscope observation in vivo. Results: Our experiments suggested a time of 24 h after the administration of PPIX to be best for ultrasound exposure. At an ultrasound intensity 5 W/cm2 and a PPIX dose 5 mg/kg, a significant synergistic effect of ultrasound combined with PPIX was observed, reducing tumor volume and increasing average animal survival time; this synergistic effect was obviously stronger than ultrasound treatment alone, while PPIX alone showed no significant effect. Transmission electron microscope observation indicated that changes in cell ultrastructure, such as cell membrane destruction, mitochondria swelling and chromatin condensation, were important factors that inhibited tumor growth and even induced cell death. Conclusion: The results implied that the antitumor effect of ultrasound could be enhanced in the presence of PPIX which might be involved in a sonochemical mechanism.