The vapor-liquid-solid (VLS) growth model has been widely used to direct the growth of one-dimensional (1D) nanomaterials, but the origin of the proposed process has not been experimentally confirmed. Here we report the experimental evidence of the origin of VLS growth. Al69Ni31 alloyed particles are used as "catalysts" for growing AIN nanowires by nitridation reaction in N-2-NH3 at different temperatures. The nanowire growth occurs following the emergence of the catalyst droplets as revealed by in situ X-ray diffraction and thermal analysis. The physicochemical process involved has been elucidated by quantitative analysis on the evolution of the lattice parameters and relative contents of the nitridation products. These direct experimental results reveal that VLS growth of AIN nanowires is dominated by the phase equilibrium of the Al-Ni alloy catalyst. The in-depth insight into the VLS mechanism indicates the general validity of this growth model and may facilitate the rational design and controllable growth of 1D nanomaterials according to the corresponding phase diagrams.