The extraction and decomposition characteristics of an organo-chlorine herbicide (4-chloro-2-methyl phenoxy) acetic acid (MCPA) absorbed on kaolin as a model soil were investigated by means of ultrasonic irradiation. It was found that a higher extraction ratio of MCPA from the model soil was obtained in a shorter time by irradiation of ultrasound of 500 kHz frequency and 21 W power, compared with the case where the ultrasound was not applied. From the experiments of ultrasonic irradiation on the slurry of MCPA-containing model soil, it was found that the concentration of MCPA extracted from the soil increased with time to attain a maximum value, then decreased with further exposure to ultrasonic irradiation. With the progress of ultrasonic decomposition of MCPA and the reaction intermediates, the concentration of Cl- released from these organo-chlorides was increased in the extract. On the other hand, it was found that the degree of ultrasonic decomposition of MCPA absorbed on kaolin was lower than that of MCPA in water. The lower decomposition efficiency of MCPA absorbed on kaolin was thought to be attributable to the presence of mass transfer resistance of kaolin particles for MCPA extraction, as well as the attenuation of ultrasound waves owing to an excessive amount of kaolin particles in the slurry.