The effects of composition distribution on microphase-separated structures formed by monodisperse BAB triblock copolymers were investigated. Monodisperse nine parent BAB triblock copolymers consisting of polystyrene for A and poly(2-vinylpyridine) for B were prepared by anionic living polymerization. These nine copolymers were designed such that polystyrene volume fraction, 0,, ranged from 0.1 to 0.9, and they were blended to produce samples with various composition distributions but with constant average composition at 0, of 0.5. Periodic simple alternating lamellar structures were observed for solvent cast and well-annealed blend films as long as composition distribution is relatively low. It has been found that microdomain spacing increases with increasing composition distribution of copolymer samples up to 1.25 in terms of M-w(S)/M-n(S), where M-w(S) and M-n(S) are weight-average and number-average molecular weights of polystyrene blocks, respectively, as was the case of the AB diblock copolymer system, though the increment is larger for triblock than diblock. Furthermore, it was clarified that triblock copolymer tends to macrophase separate into several regular microphase-separated structures more easily than diblock copolymer.