Boron nitride thin layers are in situ fabricated on chemical vapor-deposited boron in ammonia gas. Characterization by X-ray photoelectron spectroscopy and transmission electron microscopy reveals that the nitridation is dominated by different processes with varying temperatures. Below 1300 degrees C the surface reaction is in control and leads to the formation of uniformly thin layer with mostly sp3 boron nitride. As the temperature rises, the nitridation is gradually turned as a diffusion-determining process, after which a thicker but uneven layer with hexagonal sp2 boron nitride is produced.