Polyaniline (PANI) micro/nanosheets are successfully synthesized by a template-free method without using any conventional oxidants. Scanning electron microscopy, transmission electron microscopy, and FT-IR spectroscopy are applied to characterize the products. By investigating the morphologies and chemical structures of the PANI micro/nanosheets, a possible formation mechanism is proposed. In addition, the influences of experimental parameters, such as the kind of dopant, concentration of aniline, and acidity of reaction system, on the morphologies of the PANI micro/nanosheets have been systematically investigated.