We report the self-assembly of microbelts from Cu-doped boron nitride nanotubes by chemical vapor deposition using Cu as a catalyst. The BN nanotubes with Cu-encapsulated flat tips have uniform outer diameters of approximately 240 nm, inner diameters of approximately 210 nm, and lengths of approximately 10 microm, and Cu is found to uniformly dope into the nanotubes. Most importantly, the nanotubes are self-assembled to form microbelts with 10 microm in width and tens of micrometers in length. Strong and tunable light emissions from the ultraviolet to visible light range are observed in such Cu-doped microbelts, showing their high potential for applications in lasing, light-emitting diode, and medical diagnosis devices.