A novel structural induction method has been developed to synthesize cubic boron nitride (cBN) nanocrystals by the reaction of Li3N with BBr3 at very soft conditions (250℃ and 0.3-0.5Mpa). The experimental results show that at reaction temperature of 250℃, the reaction product is only hexagonal boron nitride without cubic gallium phosphide (GaP) nanocrystals being added into the reaction mixture. On the contrast, when cubic GaP nanocrystals were added to the mixture, nearly pure cBN can be obtained under the same reaction conditions. This phenomenon indicated that there was a structural induction effect of GaP to the formation of cBN. As the temperature increased, GaP nanocrystals decomposed rapidly and the structural induction effect disappeared, almost no cBN can be obtained at high temperature. Furthermore, the nucleation and growth of cBN nanocrystals were analyzed by the heterogenous nucleation theory.