The stereochemistry of radical halogenation of alkyl halides has been studied by ab initio molecular orbital theory. Two key elementary reactions, hydrogen abstraction reaction [XCH2CH3 + Y-. --> XCH2CH2. + HY (R1)] and halogen abstraction reaction [XCH2CH2. + Y-2 --> XCH2CH2. + Y-. (R2)], as well as rotational barrier of XCH2CH2. radical, with X = H, F, Cl, and Br and Y = F, Cl, and Br, were studied using the G2(MP2.SVP) theory. Reactions R1 and R2 with X = F, Cl, and Br were found to be stereoselective. For X = F, both reactions prefer a gauche abstraction, whereas for X = Cl and Br, both reactions prefer a trans pathway. The high rotational barriers of ClCH2CH2. and BrCH2CH2. radicals and the low abstraction barriers of their reactions with Cl-2 and Br-2 are the two main factors that guarantee the retention of their radical configuration during the abstraction reactions, Thus, radical chlorination and bromination of alkenes and chlorine- and bromine-substituted alkanes are predicted to be stereospecific. in good accord with experimental results. We show that the stereochemical control observed in radical halo-enation reactions can be explained without the use of Skell hypothesis. The trends of the calculated energy differences between the gauche and trans transition states of reactions R1 and R2, the rotational barriers of XCH2CH2. radicals, and the gauche effect of XCH2CH3 can be rationalized in a uniform manner in terms of hyperconjugation interaction.