Thermosetting terpolymer composed of benzoxazine (BA-a), cyanate ester (BADCy) and epoxy resin (E₄₄) was prepared via co-curing reactions. The curing procedure for the ternary blend was studied by differential scanning calorimetry (DSC) and Fourier transform-infrared spectroscopy (FT-IR), and the thermal, mechanical and dielectric properties of the terpolymer were also characterized. The DSC results manifested a multiple curing pattern, suggesting the complexity of the reactions in the system. The FT-IR analysis further showed that the curing procedure involves several steps, firstly the cyclotrimerisation of cyanate ester after oxazine ring opening, followed by the isomerization of cyanurate and the reaction of isocyanurate with epoxy to form oxazolidinone, and finally the copolymerization of the remaining epoxy with benzoxazine and the self-polymerization of benzoxazine. Compared to the polybenzoxazine, the higher crosslinking density of the terpolymer led to the higher glass transition temperature (T_g), as well as the 5% and 10% weight loss temperatures (T_d5 and T_d10), indicating the better thermal stability of the terpolymer at service temperature. And the data for the mechanical and dielectric properties also showed that the terpolymer performs better than polybenzoxazine and the copolymer.