Four high-purity aromatic diamine-based benzoxazines (13–16), which could not easily be synthesized by traditional approaches, were successfully synthesized by a facile, widely useful three-step synthetic method using four typical aromatic diamines – 4,4′-diamino diphenyl methane (1), 4,4′-diamino diphenyl sulfone (2), 2,2-bis(4-(4-aminophenoxy)phenyl)propane (3), and bis(4-(4-aminophenoxy)phenyl)ether (4), respectively, as starting materials. The structures of the monomers (5–16) were confirmed by 1 H, 13 C, 1 H– 1 H and 1 H– 13 C NMR spectra. Their high performance thermosets, P(13–16), were obtained by thermal curing of benzoxazines (13–16), and their properties were studied and compared with polymer derived from bis(3,4-dihydro-2H-3-phenyl-1,3-benzoxazinyl)methane (F-a), a typical aromatic biphenol-based benzoxazine. Among the benzoxazines, 13 and F-a are constitutional isomers, but the T g value and 5% decomposition temperature of P(13) are 53 and 111°C, respectively, higher than those of P(F-a), demonstrating the power of the molecule-approach to enhance the thermal properties. Because of the large varieties of aromatic diamines, this approach can increase the molecule-design flexibility of benzoxazines.