Block copolymer nanostructures have attracted great attention because of the wide range of applications such as sensors and drug delivery. The fabrication of block copolymer nanostructures with controlled morphologies and sizes, however, is still challenging. Here, we study the fabrication of nanotubes and nanospheres of polystyrene-block-polybutadiene (PS-b-PBD) using anodic aluminum oxide (AAO) templates. When PS-b-PBD solutions in N-methyl-2-pyrrolidone are introduced into the nanopores of the AAO templates applying the traditional solution wetting method, PS-b-PBD nanotubes can be obtained. When PS-b-PBD solutions in the nanopores are in contact with a nonsolvent, acetic acid, PS-b-PBD nanospheres are formed. Two possible mechanisms are proposed to discuss the formation of the nonsolvent-driven morphology transformation, including the Rayleigh-instability-type transformation mechanism and the nucleation and growth mechanism. The effect of the polymer concentrations on the internal morphologies of the PS-b-PBD nanostructures is discussed; at higher concentrations, PS-b-PBD nanocapsules can also be prepared. Furthermore, core-shell PS-b-PBD/polymethylmethacrylate nanospheres can be fabricated using this strategy with polymer blend solutions. This work not only demonstrates a simple strategy to control the morphologies of block copolymer nanostructures but also deepens the understanding of the interactions between polymer solutions and solvents.