Abstract: MgO-Al₂O₃-SiO₂/boron phenolic (MAS/BPF) ceramifiable composites, based on ceramifiable fillers of nano MgO, Al₂O₃ and SiO₂(MAS) modified matrix boron phenolic resin(BPF), were prepared with fusing agent of B₂O₃, Bi₂O₃ or glass frits by hot compression molding. The effects of fusing agent on the thermal stability, microstructure and phase-transition of MAS/BPF ceramifiable composites were studied. The results show that the dimensional shrinkage rate varies from 9% to 14.5% after pyrolysed at 600-1 000℃ for 2 h, indicating a great dimensional stability. And TG analysis reveals that the fusing ageing reduces the char yields of MAS/BPF ceramifiable composites at 1 200℃, but leading to a higher thermal stability at 900-1 200℃. XRD and SEM manifest ceramic reaction occurs at lower temperature with the addition of fusing agent. Compared with B₂O₃ and Bi₂O₃, glass frits contribute to forming a large amount of liquid phase structure, which promotes the reaction between the porous carbon from the decomposed resin matrix and ceramifiable fillers, improving the density and dimensional stability at high temperatures of MAS/BPF ceramifiable composites.