In this work, dense TiB2-B4C based ceramics with different amounts of hBN additions were spark plasma sintered at 1700−1850 °C through in-situ reactions involving with TiCxN1-x (x = 0.5 and 0.7) and boron. The molar Gibbs’ free energy of TiCN is lower than that of TiC and TiN, thereby the boronizing of TiCN is more favorable in multiple steps which suppressed the combustion process in the TiCN-B system. Taking advantage of this, the reaction rate in the TiCN-B system was dragged to accompany with that of densification, resulting that nanometric TiB2 grains were gradually produced and microstructures composed of B4C-hBN reinforcement and TiB2 –hBN(C) matrix were finally built. Although hBN additions reduce the Young’s modulus, TiB2-B4C composites with 17.4 vol% hBN additions densified at 1700 °C still exhibit excellent mechanical properties. The present study suggests solid solution powders are a class of promising reactants for achieving nanoceramics with better performance.