Crystalline hexagonal boron nitride (h-BN) hexagonal nanoplates were synthesized by the reaction of B2O3, NH4F, and metallic Na at 700 degrees C. The edge sizes of the hexagonal nanoplates are 400 nm, and the average thickness is 100 nm. As the reaction temperature was set at 500 degrees C, rhombohedral boron nitride (r-BN) triangular nanoplates nearly 200 nm across and about 20 nm thick were obtained. TGA analysis results reveal that thermal stability of h-BN hexagonal nanoplates was better than that of r-BN triangular nanoplates in air.