HNO3-doped poly(o-aminophenol) (PoAP) was synthesized by a facile chemical polymerization without using any surfactants or templates. Sphere-like microstructures of PoAP are obtained via self-assembling of two-dimensional nanoplates. The different microstructures of PoAP could be easily controlled by controlling polymerization conditions, and the possible formation mechanism of PoAP microstructures follows the prickly-fluffy-full sphere growth process. By susceptibility test, the as-synthesized PoAP exhibits superior and sustained antibacterial activities against the typical Gram-positive Bacillus subtilis. Transmission electron microscope, scanning electron microscopy, and antibacterial test showed that morphologies of the PoAP have an obvious influence on its antibacterial activities, and the reason was further discussed. The resultant PoAP in this study may have potential applications as biological materials. Copyright © 2014 John Wiley & Sons, Ltd.