3-D rose-like microstructures of polyaniline (PANI), which are self-assembled from 2-D nanosheets consisted of 1-D nanofibers, were synthesized by a template-free method in the presence of ammonium peroxydisulfate (APS) as both oxidant and dopant under a high relative humidity of 80% for the first time. When the relative humidity increases from 25 to 80%, not only morphology of the micro/nanostructured PANI undergoes a change from 1-D nanofibers to 2-D nanosheets to 3-D rose-like micro structures, but also increase in crystallinity. It is proposed that a cooperation effect of the oriented water molecules at the vapor-water interface and difference in hydrogen bonding energies between the interface and the bulk induced by the relative high humidity results in the formation of the 3-D rose-like microstructures self-assembled from 2-D nanosheets. Moreover, the method reported may provide a simple approach for understanding self-assembly of complex micro/nanostructures of PANI.