Density functional theory (DFT) and direct ab initio molecular dynamics (MD) calculations have been applied to a hydrogen atom trapped in diamond cluster. The DFT calculation showed that spin density of hydrogen atom in diamond (hyperfine coupling constant, hfcc) is lower than that of free hydrogen atom in vacuo. This result was in good agreement with that of muon-spin-rotation experiment. The MD calculation showed that the hfcc of hydrogen atom decreases with increasing temperature because the hydrogen atom behaves as electron donor in diamond lattice. The electronic states of hydrogen atom trapped in the diamond were discussed on the basis of theoretical results. (C) 2011 Elsevier B. V. All rights reserved.