Two structurally different bisbenzoxazine monomers (tBP-oda and tBP-jeff(148)) are synthesized and reinforced with graphene oxide (GO) at concentrations ranging from 0.25 to 3 wt %. Successful synthesis of the benzoxazine monomer and conversion from graphite to GO are verified by proton nuclear magnetic resonance spectroscopy (H-1 NMR), Fourier transform infrared spectroscopy (FT-IR), and X-ray diffraction (XRD), respectively. Dispersibility of GO in the benzoxazine monomers prior to polymerization is studied using rheological analysis, and quantified according to the theory of fractal model of colloidal gels. The polymerization behavior of the GO/benzoxazine mixtures is studied by both differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Rheological analysis is also applied to the nanocomposite precursors. Better dispersions are achieved using tBP-oda, the benzoxazine with a high degree of aromaticity in its chemical structure. The addition of GO exhibits a negative effect on the polymerization of the two benzoxazines. The mechanical properties and the glass transition temperature T-g of GO/poly(tBP-oda) nanocomposites increases, whereas for the GO/poly(tBP-jeffi(148)) nanocomposites, the mechanical properties are moderately enhanced and T-g is reduced as a function of the GO concentration. The modifications of the mechanical and thermal properties of the nanocomposites are mainly attributed to the degree of dispersion of the GO nanosheets.