This study discusses the miscibility, microstructure, and the thermal and dielectric properties of the reactive blends of dicyanate ester of bisphenol A and diamine-based benzoxazine. DSC and IR were applied to monitor the curing reactions of the reactive blends. Three reactions were observed in the curing reactions: the first is the cyclo-trimerization of cyanate ester, producing the triazine structure; the second is the coreaction between the triazine structure and benzoxazine, forming alkyl isocyanurate and diphenyl ether linkages, followed by the further reaction of the isocyanurate linkage; and the third is the ring-opening of benzoxazine. The coreaction decomposes the polycyanurate structures and brings parts of the polycyanurate structures into the polybenzoxazine matrix. Thus, a miscible blend can be achieved in all compositions, as judged by the single composition-dependent T-g in the dynamic mechanical and thermal mechanical analyses. The experimental data also show that the dimensional stability and dielectric properties are enhanced via the reactive blends.