The phase behavior of ternary homopolymer/gradient copolymer blends is Studied theoretically using a multiblock model of gradient copolymers. Critical lines and Lifshitz points of the blends are determined by a random phase approximation analysis. Phase diagrams and microphase structures of the blends are studied using self-consistent mean-field theory. It is discovered that the chain gradient distribution has a significant effect on the phase behavior of the blends. When the chain gradient distribution is gradual, a larger amount of copolymers and a higher incompatibility are needed for the formation of a microphase., In addition, the ability of copolymer domains to absorb homopolymers decreases, whereas the solubility of copolymers in homopolymer matrix increases, as the gradient distribution becomes gradual. Another important effect is that gradient copolymers lead to a broad interface between two incompatible polymers. These results indicate that interfacial structure and morphology of these blends can be fine-tuned by specifically designed gradient distributions.