Two oxyethylene/oxybutylene block copolymers (E(40)B(79) and E(47)B(62)), which exhibit body-centered cubic sphere (bcc) and hexagonally packed cylindrical (hex) melt morphologies in bulk, respectively, were blended with nanoclay of montmorillonite (MMT). The effects of MMT on the morphology and crystallization of E(40)B(79) and E(47)B(62) were studied with small-angle x-ray scattering, wide-angle x-ray diffraction, and differential scanning calorimeter. It is found that the E block in the block copolymers can intercalate into the galleries of MMT, leading to a larger layer spacing than that of neat MMT. The preferential absorption of the E block onto MMT plates induces the formation of a new lamellar structure, irrespectively of original morphology in the bulk. There is, however, coexistence of the new lamellar structure with regions retaining the melt morphology. The order-disorder transition temperature (T(ODT)) of the block copolymer is increased by MMT for E(40)B(79), but it remains unchanged for E(47)B(62). Crystallinity of the block copolymers is also greatly suppressed by the addition of MMT.