Ultrasonic decomposition of (4-Chloro-2-methyl phenoxy)acetic acid (MCPA) in aqueous solution was performed at a frequency of 500 kHz under an argon atmosphere. We investigated the mechanism of ultrasonic degradation of MCPA by observing the decomposition time evolution of MCPA and the behaviors of chloride ion, TOC (total organic carbon), and byproducts formed through the ultrasonic degradation process of MCPA.At a constant power of 21.4 W, MCPA was completely decomposed after 180-min sonication and the ultrasonic decomposition of MCPA followed a pseudo first order reaction kinetics. Much of its chlorine atoms were mineralized in solution after 360-min sonication. On the other hand, it was difficult to achieve complete mineralization of carbon, and about 60% of the initial TOC remained in aqueous solution after 360 min. Based on the GC-MS analysis, byproducts such as 4-chloro-2-methyl phenol (CMP), 4-chlorocathecol, methylhydroquinone, cresol, acetic acid and formic acid were detected in the aqueous solution to which ultrasonic irradiation was applied. The CMP concentration increased during the early period of irradiation reaching a maximum concentration at about 60 min followed by its decrease until CMP became undetectable at 180 min. Decomposition of MCPA by ultrasonic irradiation resulted in the increase in acetic acid concentration until 120 min, after which no further increase was observed. Conversely, formic acid concentration was observed to increase only during the duration of sonication experiment.