The effects of the composition of added C-b-D diblock copolymer (f = N-c/N, where N is the total chain length of block copolymer and Nc that of block C) on the dynamics of phase separation of an immiscible binary blend A/B were investigated by Monte Carlo simulation. The composition of the block copolymers has an important effect on the phase separation, which is also strongly dependent upon the chain lengths of the homopolymers and block copolymer. For block copolymers with chain lengths shorter than the homopolymers, a symmetric composition (f = 0.5) is most effective in slowing the growth rate of phase-separated domains. However, for block copolymers with chain lengths longer than the homopolymers, an asymmetric composition almost identical with the composition of the homopolymer blend (A/B = 75/25) has the best retardation effect because of the curvature properties of the interface, i.e., the elasticity and spontaneous radius of curvature of the interface. The scaled structure factors were calculated and analyzed for A/B/C-b-D blend systems containing asymmetric block copolymers. The universality of the scaled structure function at reduced wave number x < 2, where x = q/q(1)(t) and q(1)(t) is the first moment of structure factor, indicates that the global structure of the phase-separated domains has a dynamical self-similarity independent of the composition of the block copolymers.