Polyaniline (PAni) mesostructures have been synthesized under hydrothermal conditions. The mesostructures show different forms-fibers, dendrite fibers, textured plates, featureless plates, and spheres. The obtained morphologies are more sensitive to the concentration of the doping acid, HCl, than the reaction temperature. Similar morphological evolutions have also been observed in the cases of phosphoric (H3PO4) and fluoroboric (HBF4) acids. The measured results of UV-vis and Fourier transform IR spectroscopy, as well as X-ray diffraction data, suggest that the morphology evolution is related to the charged property and reactivity of the semiquinone radical cation on the main chain of PAni, formed by polymer doping. Hydrothermal synthesis by tuning the concentration of the doping acid provides a platform for understanding the nature of the polymerization and the fabrication of 1- and 2D conjugated polymers.