A new approach for the synthesis of polyaniline (PANI) nanostructures under UV light illumination has been developed, which is the first report of a templateless chemical process for preparing pure PANI nanowires. The acceleration effect of photo-assistance on the polymerization can promote the homogeneous nucleation and elongation of the nanofibers and nanowires, leading to easy preparation of tunable diameters of the nanowires and nanofibers of PANI.