The effect of molecular weight distribution on micropbase-separated structures for both AB diblock and BAB triblock copolymers was investigated in comparison with that of composition distribution. Monodisperse poly(styrene-b-2-vinylpyridine) (SP) and poly(2-vinylpyridine-b-styrene-b-2vinylpyridine) (PSP) parent block copolymers were synthesized by living anionic polymerizations whose volume ratios were all designed to be 0.5/0.5. Three parent copolymers were blended variously with both number-average molecular weight and composition kept constant but having different molecular weight distribution. Microphase- separated structures of sample films obtained by solvent-casting followed by annealing were observed by transmission electron microscopy and small-angle X-ray scattering. It has been found that both SP and PSP block copolymers show simple lamellar structures even when the molecular weight distribution is relatively wide and that lamellar domain spacing increases with increase in polydispersity index of the blend system. Furthermore, the increment for PSP triblock is larger than that for SP diblock as was the result for the study on composition distribution. The microdomain expansion can be caused by the localization of polydisperse block chains in both phases, which was commonly observed for both the composition distribution system and the molecular weight distribution system.