Boron nitride nanotubes (BN-NTs) with pure hexagonal BN phase have been synthesized by heating ball-milled boron powders in flowing ammonia gas at a temperature of 1200 degrees C. The as-synthesized products were characterized by X-ray powder diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, and electron energy loss spectroscopy (EELS). The diameters of nanotubes are in the rage of 40-120nm and the lengths are more than 10 mu m. EELS result identifies that the ratio of boron and nitrogen is almost 1:1. The growth temperature is a crucial growth parameter in controlling the structure and crystalline of BN-NTs. The nanotubes grown at 1100 degrees C possesses of a bamboo-like structure, while as the temperature increased to 1200 degrees C, most of the nanotubes exhibited a cylindrical structure. In addition, changing the heating time can control the size of the nanotubes. The gas atmosphere has influence on the yield of BN-NTs during heating process. When heating atmosphere was replaced by nitrogen, the yield of nanotubes was remarkably decreased.