An in situ synthesis method to fill boron nitride (BN) nanotubes with metals or metal alloys was proposed using a vapor-liquid-solid catalytic growth mechanism. Nickel fillings could be obtained by using nickel foil or a silicon substrate covered with nickel film during catalytic growth of pure BN nanombes. The filled nickel nanowires usually connect to nanoparticles at the tip-end of BN nanotubes. Extensive structural investigations display the general existence of 4-fold superstructures along the [100] and [110] axes. The superstructures ensure the existence of one set of {223} planes of the cubic nickel parallel to the BN sheet, its planar distance are four times the interplanar spacing of the BN nanombes shell, and the Ni-Ni distance in this plane is equal to the B-B and N-N distance. The (223) plane grows directly on the (0002) plane of the BN tubular layer according to a lattice-match relationship between the BN shell and the cubic nickel. NiSi2 nanowires could also fill BN nanotubes when using a silicon substrate covered with a nickel film. The alloy nanowires have the growth axis along the [0-11] direction and the (-422) planes grow parallel to the innermost BN shells. The grown plane has the three times planar distance of the BN (0002) shells, and the atomic distance in this plane accords with B-B and N-N bonds. All the results indicate that metal Ni and NiSi2 crystallizes on the innermost shell of the BN nanotubes at a specified plane restricted by the all-space lattice-match relationship. Investigations further confirm the nanometallurgical behavior: crystallization of liquid metals is strongly confined by BN nanotube geometry.