Ni-catalyzed cross-coupling of unactivated secondary alkyl halides with alkylboranes provides an efficient way to construct alkyl–alkyl bonds. The mechanism of this reaction with the Ni/L1 (L1=trans-N,N′-dimethyl-1,2-cyclohexanediamine) system was examined for the first time by using theoretical calculations. The feasible mechanism was found to involve a NiI–NiIII catalytic cycle with three main steps: transmetalation of [NiI(L1)X] (X=Cl, Br) with 9-borabicyclo[3.3.1]nonane (9-BBN)R1 to produce [NiI(L1)(R1)], oxidative addition of R2X with [NiI(L1)(R1)] to produce [NiIII(L1)(R1)(R2)X] through a radical pathway, and CC reductive elimination to generate the product and [NiI(L1)X]. The transmetalation step is rate-determining for both primary and secondary alkyl bromides. KOiBu decreases the activation barrier of the transmetalation step by forming a potassium alkyl boronate salt with alkyl borane. Tertiary alkyl halides are not reactive because the activation barrier of reductive elimination is too high (+34.7 kcal mol−1). On the other hand, the cross-coupling of alkyl chlorides can be catalyzed by Ni/L2 (L2=trans-N,N′-dimethyl-1,2-diphenylethane-1,2-diamine) because the activation barrier of transmetalation with L2 is lower than that with L1. Importantly, the Ni0–NiII catalytic cycle is not favored in the present systems because reductive elimination from both singlet and triplet [NiII(L1)(R1)(R2)] is very difficult.