Synthesis of boron nitride spheres (BNS) was achieved by vapor phase pyrolysis of ammonia borane (BH3NH3) using two independently temperature-controlled furnaces in a glove box filled with N-2. The BNS were heated at 1300 degrees-1700 degrees C in flowing NH3, N-2, or Ar by multistep heat treatment. The sizes of the BNS could be controlled by heating BH3NH3 at different rates (1 degrees C/min: 300-800 nm, 5 degrees C/min: 300 nm-1.2 mu m and 10 degrees C/min: 300 nm-1.8 mu m). The microstructures of BNS heated in different ambient gases were observed using transmission electron microscopy with selected area electron diffraction. NH3 gas produced BNS with well-crystallized surface shells and amorphous cores whereas N-2 and Ar gases crystallized the entire BNS. It is evident that of these three ambient gases (NH3, N-2, and Ar), N-2 gas significantly enhances the crystallization of BN with randomly oriented grains.