The effects of hydrogen on Ni homoepitaxy have been investigated by examining rate constants for mobility of Ni during the Ni-growth process with and without the presence of a small amount of hydrogen impurity which is found to act primarily as a catalyst. The rate constant activation energies are determined by classical-potential total-energy calculations with semiclassical zero-point energy corrections for the hydrogen atom. This work extends previous work in which we have examined the effects of H on submonolayer (two-dimensional) Ni(100) growth in Haug, K.; Zhang, Z.; John, D.; Waiters, C. F.; Zehner, D. M.; Plummer, W. E. Phys. Rev. B. 1997, 55, R10233-R10236 and Haug, K.; Do, N. K. N. Phys. Rev. B. 1999, 60, 11095. We find that fast diffusion of H atoms occurs on the flat Ni surfaces and the presence of these highly mobile H atoms is found to have significant effects on the mobility of lone Ni adatoms on Ni(100), but not on the Ni(110) and Ni(lll) surfaces. The H atoms are also found to have significant catalytic effects lowering the step edge (Ehrlich-Schwoebel descent) barrier and therefore accelerating breakdown of three-dimensional Ni islands which form during the epitaxial growth on the Ni(100) surfaces, and to a very modest extent on the Ni(lll) sui-face, but this net catalytic effect is not found on the Ni(110) surfaces. While the overall H atom effects on the Ni mobility in these cases are primarily catalytic, the kinetically determined Ni island morphologies may differ substantially over time periods which are long on the deposition time scale, and therefore the morphology differences can become frozen in place.