The structural features and microhardness of sintered at high pressure and high temperature with the addition of nano-and micron-sized powders of cubic BN (cBN) self-fluxing nickel alloy PG-SR4 are studied. Based on the experiments, the modes of high pressure and high temperature treatment are established. The optimal content of the nanostructured cBN additive is determined, at which the material with the highest microhardness is formed. It is shown that the microhardness of the material with the addition of micron-sized cBN is 1.5-2 times lower than that for the samples with the same content of nanostructured cBN.