The preparation of the fine BN powders by ammonia-nitridation of ammonia-decaborane derivate was attempted at temperature between 300 to 150$0^{circ}C$. The formation mechanism of BN was examined and the resulant BN powder was characterized by means of IR, XRD, SEM and PSA method. In the nitridation below 80$0^{circ}C$ bonding materials were identified with mainly BH and NH but above 80$0^{circ}C$ with BN by IR spectra and X-ray patterns. Crystalite size, lattice constant and particle size distribution of hexagonal BN prepared at 1, 50$0^{circ}C$ were La=470$AA$, Lc=180$AA$, a=2.5062$pm$1$AA$, c=6.8285$pm$2$AA$ and 2.0-5.4${mu}{ extrm}{m}$, respectively.