The polycrystalline diamond compact (PDC), which consists of a polycrystalline diamond layer on a tungsten carbide (WC)/cobalt (Co) substrate, is extensively utilized as drilling bits. However, the poor thermal stability due to the graphitization and oxygen susceptibility of diamond severely limits the application of PDCs to high-temperature drilling work. In this study, a new PDC with improved thermal stability is successfully synthesized with boron (B)-coated diamond particles, which forms a uniform boron carbide (B4C) barrier. The as-received B4C phase acts as a protective barrier, which enhances the initial graphitization and oxidizing temperatures to 800 °C and 780 °C, respectively, which are ~100 °C and ~30 °C higher than those (700 °C and 750 °C) of the PDC sintered with uncoated diamond particles. The B4C barrier protects diamond grains from direct contact with the Co phase, prohibiting the cobalt-catalytic graphitization. In addition, the oxidation of the B4C barrier occurs prior to that of the diamond grains, which inhibits the PDC from oxidation.