A phase diagram as a function of molecular weight ratio and blend composition was constructed for the binary blends of nearly symmetric polystyrene-block-polyisoprene (PS-PI) copolymers. In this study we prepared four series of binary blends, and every blend consists of a copolymer, hereafter designated as alpha, whose number-average molecular weight (M-n) and volume fraction of PS-block (f(PS)) are 1.0 x 10(5) and 0.47, respectively. Four PS-PI copolymers (hereafter designated beta), each of which is blended with alpha, have their M-n and f(PS) in the range 1.2 x 10(4)-2.1 x 10(4) and 0.40-0.49, respectively, and therefore the molecular weight ratio of alpha to beta, hereafter designated as r, ranges between 4.8 and 8.3. All the blends showed partial miscibility of the constituent copolymers; that is, at a certain blend composition the macrophase separation between alpha and beta occurred. However, the range of blend compositions where the constituent copolymers phase-separate became small as r decreased. As for the two blend series where r = 4.8 and 6.9, the PS cylindrical morphology was observed over wide blend compositions, although the constituent copolymers have nearly symmetric compositions and by themselves are self-assembled into lamellar morphology.