Monte Carlo simulation was used to study the effects of C-b-D block copolymers with repulsive interaction on the phase separation dynamics of A/B immiscible homopolymer blend. It is found that a repulsive block copolymer can significantly suppress the rate of phase separation of an immiscible binary blend when its interaction energies with the homopolymers satisfy a proper condition. It is also found that the longer the chain length of block copolymer, the better such a retardation effect. When the chain extension of block copolymer at the interface is examined, each block in the repulsive block copolymer is contracted in its corresponding homopolymer phase, whereas the attractive copolymer chain is extended at the interface. This may lead to less retardation effect of repulsive block copolymer as compared with attractive block copolymer. Unlike the attractive block copolymer, the repulsive block copolymer with an asymmetric composition (f = 0.7) does not show better retardation than the symmetric one. However, in spite of some difference in retardation effect, both types of block copolymers (attractive and repulsive) obey the dynamic scaling law, and the scaled structure factors are universal with time, irrespective of copolymer structure.