© Copyright 2018 American Chemical Society. Chemical vapor deposition (CVD) of two-dimensional materials has been an active area of research in recent years because it is a scalable process for obtaining thin films that can be used to fabricate devices. The growth mechanism for hexagonal boron nitride (h-BN) on metal catalyst substrates has been described to be either surface energy-driven or diffusion-driven. In this work, h-BN is grown in a CVD system on Ni single-crystal substrates as a function of Ni crystallographic orientation to clarify the competing forces acting on the growth mechanism. We observed that the thickness of the h-BN film depends on the Ni substrate orientation, with the growth rate increasing from the (100) surface to the (111) surface and the highest on the (110) surface. We associate the observed results with surface reactivity and diffusivity differences for different Ni orientations. Boron and nitrogen diffuse and precipitate from the Ni bulk to form thin multilayer h-BN. Our results serve to clarify the h-BN CVD growth mechanism which has been previously ascribed to a surface energy-driven growth mechanism.