Titanium-coated boron nitride nanotubes prepared by sputtering method have been heated under different conditions to investigate reaction phases at their interface. It is found that TiB 2 nanocrystals firstly nucleate on the surface of nanotubes at 800 °C for 30 min. By the time 1 h, these nucleated TiB 2 crystals grow larger by consuming inner walls of nanotubes and tend to transform into TiB phase. With an exposure to higher temperatures and longer periods, needle-like TiB whiskers and plate-like TiB 2 remain in the composite.