This paper shows that if G = H x K is a semidirect product of finite groups, then Aut G = C(Aut G)(H) C(Aut) (G)(K) if and only if theta(K) boolean AND H = 1 and [K,theta] subset of C(G)(H) for all theta is an element of Aut G. As an application, we investigate the automorphism group of a split metacyclic p-group for odd p.