Monte Carlo simulations were used to model A/B/triblock copolymer ternary mixtures with a fixed volume fraction (19%) of homopolymer A. The volume fraction of black copolymer was varied from 0 to 66%. The simulation illustrated how two incompatible polymers A and B can be blended with a black copolymer. When homopolymer A forms a dispersed phase, both B-A-B and A-B-A triblock copolymer chains tend to stretch on the interface, and some of them form the interchain association an the interface, so that triblock copolymers are more efficient as compatibilizers than A-B diblock copolymers. Some of A-B-A triblock copolymer chains are not located at the interface, but form polymeric micelles in the matrix phase, while nearly all the triblock copolymer chains are concentrated at the interface. Therefore, B-A-B triblock copolymers are better compatibilizers.