High melting temperature and viscosity of most benzoxazine monomers are challenging problems in processing, especially for preparation of composites. The present work introduces the "reactive diluent" concept to improve the processability of benzoxazines. A benzoxazine monomer having a vinyl group is first synthesized from 4-vinylguaiacol (4VG), and then dissolved in a reactive diluent, styrene or methyl methacrylate. The resulting solutions show low viscosity (less than 2 mPa s), and exhibit high reactivity during free radical and ring-opening polymerizations, successively. Furthermore, the solutions are applied to fabricate cotton fabrics reinforced composites. Both of them possess high glass transition temperature and excellent mechanical properties, which have potential applications as structural components.