The preparation of dense Si₃N₄-based ceramics has attracted great attention because of the achievable improvements in their mechanical properties and high-temperature oxidation resistance. In this work, advanced dense boron-containing α/β-Si₃N₄/Si monoliths were prepared via a high pressure‒high temperature technique in which polymer-derived amorphous SiBN powders were used 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 °C. The results demonstrate that the incorporation of boron in the Si₃N₄ matrix suppresses the 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 reached 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 °C. In addition, the oxidation behavior of the samples prepared at 1400 and 1600 °C was investigated at 1400 °C for 50 h. The results show that the incorporation of boron significantly improved the oxidation resistance of the samples because of the formation of borosilicate/cristobalite. This work provides guidance for the synthesis of boron-containing α/β-Si₃N₄-based ceramics with excellent mechanical properties and oxidation resistance.