Hydrogenation of single-crystal yttrium (Y) thin films was studied with nuclear reaction analysis (NRA), and low energy electron diffraction (LEED). The Y(0001) thin film with 20 nm thickness was epitaxially grown on W(110). Hydrogenation was carried out by exposing the Y films to molecular and atomic hydrogen. Absorption rate of hydrogen under exposure to molecular hydrogen was the same as that to atomic hydrogen at 700 K indicating that the dissociation of H₂ is not the rate-determining step. The hydrogen concentration was found to saturate at H/Y=1.7, and the trihydride γ-YH₃ phase was not formed above 450 K in 10⁻³ Pa H₂. At a low temperature (≈100 K), on the other hand, the dihydride (β-YH₂) formation was suppressed, and only the α-phase was found to grow. The depth profile of the absorbed hydrogen indicates that the absorbed hydrogen is distributed uniformly in the depth direction of the Y thin films.