The second-order activation rate constants k(A) for low-molar-mass alkyl halides catalyzed by cuprous halide complexes Cu(I)X/2L (X = Cl or Br; L = 4,4'-diheptyl-2,2'-bipyridine) were determined by the nitroxide capping method along with H-1 NMR, The k(A) for 1-phenylethyl bromide, a typical initiator for atom transfer radical polymerization (ATRP), with the Cu(I)Br complex was found to be close to the known value of the k(A) for a polystyryl bromide, being large enough for the initiation to be completed at an early stage of polymerization. It was also found that k(A) strongly depends on the kind of halogen and the steric factor of the alkyl halide in question.