It is pointed out theoretically that the ordered morphology in the binary mixtures of A-B type diblock copolymers, ((A-B)(1)/(A-B)(2)), depends not only on the compositions of block copolymers (i.e., the volume fraction of the A block, f(A1),f(A2)) but also on the relative chain-length differences of the constituent copolymers (alpha (j) for jth (j = A or B) block where alpha (j) is defined by alpha (j) = (N-j(2) - N-j(1))N-j(1) and N-j(i) (i = 1 or 2) denotes the polymerization index of the jth block chain in the ith block copolymer (A-B)(i)) and blend composition (i.e., the mole fraction of(A-B)(2) having a larger molecular weight, q(2)), whereas the morphology in the single component A-B diblock copolymer system in a strong segregation limit solely depends on the composition (f(A)) We explored ordered morphology of binary mixtures of lamella-forming polystyrene (PS)-block-polyisoprene (PI), (PS-PI), with given block compositions (f(PS1) = 0.44, f(PS2) = 0.46) and given relative chain-length differences (alpha (PS) = 6.18, alpha (PI) = 5.68) as a function of q(2). Our results revealed both the regime where only microphase separation occurred (q(2) = 0 and 0.179 less than or equal to q(2) less than or equal to 1) and the regime where macrophase separation occurred as well (0.003 less than or equal to q(2) less than or equal to 0.127). In the microphase separation regime, we found a unique morphological transition between lamellar and cylindrical morphology with q(2) whose trend is consistent with the theoretical prediction by Lyatskaya et al. (Polymer 1992, 33, 343). In the macrophase separation regime we found the two coexisting macrophases with cylindrical and lamellar morphology. The miscibility gap of the two PS-PIs, with respect to q(2) for the macrophase separation, was found to be consistent with a theoretical prediction by Matsen.