The reaction of atomic O (P-3) with CH3CHF2 has been studied theoretically using ab initio direct dynamics methods for the first time. This reaction takes place through two possible channels: H abstraction from the methyl group (CH3) and H abstraction from the methyne group (CH). Two nearly degenerate saddle points of (3)A" and (3)A' symmetries have been located for each hydrogen abstraction channel. At the QCISD(T)/6-311+G(3df,2p)//MP2/6-311G(d,p) level, the potential barrier of H abstraction from the CH3 group is about 5 kcal/mol higher than that of H abstraction from the CH group. Changes of geometries, generalized normal-mode vibrational frequencies, and potential energies along the reaction paths for all the channels are discussed and compared. On the basis of the ab initio data, the rate constants of each channel have been deduced by canonical variational transition-state theory (CVT) with small-curvature tunneling (SCT) correction method over a wide temperature range of 200similar to3000 K. The theoretical results have been compared with available experimental data. The kinetics calculations show that the variational effect is small, and in the low-temperature range the small curvature tunneling contribution is important for all the channels. The detailed branching ratios have been discussed.