The hydrogen abstraction reactions by OH radical from 1,1-dichioroethane and 1,2-dichloroethane have been investigated by ab initio molecular orbital theory. Optimized geometries and harmonic vibrational frequencies have been calculated for all reactants, transition structures, and products at the (U)HF/6-311G(d,p) and (U)MP2=full/6-311G(d,p) levels of theory. Single point QCISD(T)/6-311G(d,p)//(U)MP2=full/6-311G(d,p) calculations have also been carried out for the inclusion of higher order electron correlation. Three distinct transition structures have been located for the H3C-CHCl2 + OH reaction (one for alpha-abstraction and two for beta-abstraction). Four transition structures have been located for the reaction ClH2C-CH2Cl + OH. The calculated barrier heights, reaction enthalpies, and change in entropy are found to be in good agreement with available experimental values. In addition, the rate constants calculated by using the transition state theory are found to be in good agreement with the experimental results. Non-Arrhenius behavior of the rate constants arises from the tunneling effects and availability of multiple reaction channels.