We consider finite-size effects for the dyonic giant magnon of the type IIA string theory on AdS 4 Â CP 3 by applying the Lüscher -term formula which is derived from a recently proposed S matrix for the N ¼ 6 super Chern-Simons theory. We compute explicitly the effect for the case of a symmetric configuration where the two external bound states, each of A and B particles, have the same momentum p and spin J 2 . We compare this with the classical string theory result which we computed by reducing it to the Neumann-Rosochatius system. The two results match perfectly.