Through Ab initio pseudopotential calculation, we study the magnetic property of C/BN superlattice nanotube. Generally the atomic-like localized states exist at the zigzag border and the partially filling electrons induces spin-polarization at the border. Especially the direct exchange interaction among localized states makes $C_1(BN)_1$ nanotube have itinerant ferromagnetism. We propose these superlattice nanotubes as the spin-polarization junction or the nanotube magnet.