The oxidation mechanism of monochloroacetic acid (CH2ClCOOH) by OH radical has been systematically investigated employing quantum mechanical methods coupled with kinetic calculation using canonical variational transition state theory. Three distinct transition states were identified for the titled reaction, two corresponding to the hydrogen atom abstraction and one corresponding to the chlorine atom abstraction. The rate constants of the titled reactions are computed over the temperature range 278-350 K, and the branching ratios calculated for the hydrogen atom abstraction from the -C(O)OH site and the -CH2Cl site are 25 and 75%, respectively, at 298 K. The computed branching ratio indicates that the kinetically favorable reaction is the hydrogen atom abstraction from the -CH2Cl site resulting in the formation of CHClC(O)OH radical, which further undergoes secondary reaction with O-2 and other atmospheric species. The calculated overall rate constant for the hydrogen atom abstraction reactions is in consistent with the reported experimental rate constant. The atmospheric lifetime of CH2ClCOOH is found to be around 18 days.