The preparation of dense Si₃N₄-based ceramics has attracted great attention due to the achievable improvements in mechanical properties and high-temperature oxidation resistance. In this work, advanced dense boron-containing α/β-Si₃N₄/Si monoliths were prepared by high pressure-high temperature technique using polymer-derived amorphous SiBN powders as raw materials. The crystallization behavior and phase transformation of the polymer-derived amorphous samples were studied in the temperature range from 1400 ℃ to 1800 ℃. The results demonstrate that the incorporation of boron in the Si₃N₄ matrix suppresses phase transformation from α-Si₃N₄ to β-Si₃N₄. Furthermore, the mechanical properties of the as-prepared samples were measured and the maximum hardness and fracture toughness of boron-rich SiBN samples reaches up to 14.8 GPa and 7.96 MPa·m^1/2, respectively. The hardness of the obtained boron-rich SiBN samples is stable up to 300 ^oC. In addition, the oxidation behavior of the samples prepared at 1400 ℃ and 1600 ℃ was investigated at 1400 ^oC for 50 h. The results show that the incorporation of boron significantly improves the oxidation resistance of the samples due to the formation of borosilicate/cristobalite. This work provides a guidance for the synthesis of boron-containing α/β-Si₃N₄-based ceramics with excellent mechanical properties and oxidation resistance.