ABSTRACT A six-degrees-of-freedom, time-dependent quantum dynamics calculation is employed to study the integral cross sections, full-dimensional cumulative reaction probabilities and full-dimensional rate constants for the isotopic reactions of the OD and OH with CH3 reactions. The full-dimensional cumulative reaction probabilities and full-dimensional rate constants are obtained using the energy and J-K shifting approaches based on the six-degrees-of-freedom calculations. The comparison of integral cross sections shows that the OD + CH3 reaction has a larger energy threshold and a smaller tunnelling effect than the OH + CH3 reaction. The corrected rate constants using the experimental zero-point energy have a very good agreement with the experimental results. The comparison of the rate constants shows that the OD + CH3 has smaller rate constants than the OH + CH3 reaction, which indicates a smaller reactivity due to the isotope substitution.