Abstract: The infiltration mechanism of cobalt phase in the polycrystalline diamond compacts (PDC) by the ultra-high pressure and high temperature sintering was investigated, and the effect of cobalt concentration gradient on the mechanical properties of PDC was analysed. In the results, the different assembly methods result in the liquid cobalt permeating into the diamond layer, and the obvious concentration gradient appears in the cemented carbide matrix along the interface. For the PDC obtained by the assembling method with the middle of the matrix placed in the center of high temperature region, the mass fraction of cobalt phase reaches 9.42% in the interface of the cemented carbide, much higher than that of 5.58% for the PDC obtained by the conventional assembly method, which inhibits the serious cobalt loss of the cemented carbide near the interface, the cobalt concentration gradient decreases, and the impact resistance of the PDC obtained by this assembly method is much better than that of the conventional assembly method. The cobalt concentration gradient does not affect the PDC wear resistance.