Abstract: To investigate effect of flux on properties and microstructure of resin matrix composites at elevated temperature, glass frit (GF) and Si₃N₄ modified high silica glass fiber reinforced boron phenolic resin composites (GF-Si₃N₄/BPR) were prepared via a compression molding technique using low melting point GF as flux and Si₃N₄ particles as high temperature resistant fillers. The influence of GF on the high temperature properties and dielectric properties of composites was studied. The results show that the introduced GF promotes the formation of liquid phase on the surface of composites and the densification of the ceramic layer, inhibiting erosion of composites by oxygen at elevated temperatures and significantly improving the high temperature performance of composites. The flexural strength of GF-Si₃N₄/BPR treated at 1200℃ was increased by 81.3% and 14.9%, respectively, compared with high silica glass fiber reinforced boron phenolic resin composites (BPR) and Si₃N₄ modified high silica glass fiber reinforced boron phenolic resin composites (Si₃N₄/BPR), while the mass ablation rate was reduced by 73.1% and 55.1%, respectively, compared with BPR and Si₃N₄/BPR. Furthermore, at 8.2 GHz, the dielectric constant (ε) and loss tangent (tanδ) of the composites gradually increased with increasing temperature. At temperatures above 800°C, the resulting glass phase effectively restrains the adverse effects of free carbon, pores, and cracks generated by resin cracking on the dielectric properties of the material. The prepared composite material has excellent high temperature properties and dielectric properties, and is expected to be applied in the field of high temperature wave transmission.