Organic materials such as porphyrin are decomposed by ultrasonic irradiation. In this study, the effect of liquid mixing on the performance of porphyrin decomposition by ultrasonic irradiation is experimentally investigated. The ultrasonic frequency used was 22.8 kHz. In order to create the various liquid mixing conditions, the rotation speed of the stirrer or liquid circulation flow rate is changed. Decomposition conversion increases with increasing rotation speed of the stirrer or liquid circulation now rate. Under both mixing methods, the ratio of decomposition conversion, which is defined as the ratio of decomposition conversion with liquid mixing to that without liquid mixing, is found to be correlated by the liquid mixing time.