The molecular-level design and controlled synthesis of materials are of key importance for advancing their applications. Based on the special structure of 3-aminophenol, containing both phenol and amine groups, a facile synthesis of highly monodisperse poly(benzoxazine-co-resol)-based polymer spheres was first reported by the autocatalysis polymerization of 3-aminophenol and formaldehyde without using any catalyst, surfactants, templates, and/or functional dopants at low temperature. The sizes of polymer spheres can be widely tuned from 372 to 1030 nm by changing the initial reaction temperatures and the concentrations of monomers. Based on FTIR, NMR, XPS, and EDX analysis, 3-amoniaphenol was evidenced not only to participate in the polymerization and form the structure of polybenzoxazine but also to catalyze the polymerization. Furthermore, they can be pseudomorphically and uniformly converted to the corresponding carbon spheres in high yield due to the excellent thermal stability of 3-aminophenol-formaldehyde resin. © 2018 American Chemical Society.