The phase diagram of the blends of a diblock copolymer (A-B) and a random copolymer (ABR) is constructed by the modified confined-chain model developed earlier where the morphology of A-B block copolymer is lamellae and the composition of ABR random copolymer is close to that of A-B. These theoretical results predict a relatively simple phase behavior for the A-B/ABR blends in comparison to the A-B/A blends, A being a homopolymer, studied previously. Unlike in A-B/A blends, the order-disorder transition temperature in A-B/ABR blends is lowered by the addition of the random copolymer regardless of its molecular weight. It is also found that the fraction of ABR solubilized into the A-B is very limited due to the endothermic mixing interaction between ABR and each block of A-B.