The phase behaviors of the blends of diblock copolymers and random copolymers were investigated with synchrotron small-angle X-ray scattering, light scattering, differential scanning calorimetry, and transmission electron microscopy techniques. The temperature, T-ODT, at which the order-disorder transition (ODT) takes place, was obtained as a function of the fraction of random copolymers. The phase diagram shows three regions: homogeneous liquid phase, ordered mesophase, and a two-phase region. When the fraction of random copolymer is lower than 0.15, T-ODT decreases linearly as the random copolymer fraction increases. Above 0.15, the macroscopic phase separation is dominant, and T-ODT is nearly constant in the middle range of the fraction. We discussed phase behavior on the basis of the free energy change on mixing of a diblock copolymer and a random copolymer.