Polyaniline nanostructures (nanosheets, nanofibers, and nanoparticles) can be assembled at the organic/aqueous interface or in solution by controlling the diffusion rate and the polymerization induction time of aniline. The quality of polyaniline nanostructures is determined by the polymerization solution conditions. Polyaniline nanosheets formation mechanism was proposed. Under certain polymerization conditions, polyaniline nanofibers or/and nanoparticles were obtained.