One-dimensional polyaniline nanostructures with controllable surfaces and diameters have been synthesized using vanadic acid as the oxidant in the absence of porous templates and structural directing molecules. The synthetic parameters, such as the concentrations of HCl, and types of oxdiants on the morphologies, diameters, and molecular structures of one-dimensional polyaniline nanostructures have been investigated. As the concentration of HCl is low, polyaniline nanorods with diameters of 10-20 nm and length up to : 50-60 nm can be aligned radially along the length axis on the surfaces of polyaniline nanofibers. With the concentration of HCl increased, the diameters of one-dimensional polyaniline nanostructures decrease from 150-300 to 40-100 nm, and the surfaces of one-dimensional polyaniline nanostructures become smooth. The possible formation mechanism of one-dimensional polyaniline nanostructures using vanadic acid as the oxidant has been discussed.