Three kinds of hBN(hexagonal BN), amorphous hBN with $B_2O_3$, amorphous hBN without $B_2O_3$ and highly crystllized hBN without $B_2O_3$, were prepared to reveal the effect of $B_2O_3$ on the crystallization of amorphous hBN and the effect of crystallinity of hBN on the cBN(cubic BN) synthesis under high pressure and high temperature(HP/HT). The amorphous hBN with $B_2O_3$ could easily be crystallized and transformed to cBN in the presence of AIN catalyst, while the amorphous hBN without $B_2O_3$ could not be transformed to cBN. Even without $B_2O_3$, however, well-crystallized hBN could be transformed to cBN under the same condition. It is thus found that the crystallinity of hBN determines directly the transformation of hBN to cBN: The cBN can be synthesiszed only from the crystalline hBN. When $B_2O_3$ or other additives which can facilitate the transformation of amorphous hBN to crystalline hBN are present, however amorphous hBN can eventually also be transformed to cBN.
