The B depth profiles in B-doped a-SiC: H/undoped a-Si: H heteroiunctions have been measured by utilizing the nuclear reaction 11B(p, α)8Be. From the change of B depth profiles, we estimate the coefficients of B diffusion in a-Si:H during preparation and post-deposition annealing. The behaviors of electrically activated B diffusion have also been investigated by conductivity measurements. Based on the recent studies of thermal equilibrium defects in a-Si: H, a simple discussion on the mechanism of B diffusion in a-Si:H is presented in this paper as well.