We have investigated the crystallization process of an amorphous Si layer grown on a clean and hydrogen-terminated Si(001) surfaces by ion beam induced epitaxial crystallization (IBIEC). In this study, we quantitatively clarify the influence of interfacial H amount to the IBIEC rate of the deposited amorphous Si layer. We initially prepared a typical H-free pristine Si(001)-(2×1) surface and two kinds of chemically H-terminated surfaces of as-treated and sequentially annealed at 350°C in ultrahigh vacuum, corresponding to high (dihydride) and low (monohydride) hydrogen coverage, respectively.