A qualitative understanding of the incorporation of hydrogen atoms in forsterite and corresponding octahedral vacancies are given from the inspection of the crystal structure with the view point of minimum strain configuration. In the case of Fe-free hydrous forsterite, the structural strain from the ideal size is larger in the M1 site than in the M2 site. Two H atoms may replace the Mg atom predominantly at the M1 site to reduce the structural strain, giving the vacancy at the M1 site. In the case of Fe-bearing hydrous forsterite, two H atoms may replace the Mg atom predominantly at the M2 site to avoid the combination of Fe in M1 site and Mg in M2 site for the reduction of the structural strain, giving the vacancy at the M2 site. In both cases, the most probable H atom locations are near to the edges of MO₆ octahedra. Variation of the unit cell dimensions (∂b/b₀ > ∂c/c₀ > ∂a/a₀) with the incorporation of hydrogen atom is interpreted in terms of retractable and unretractable chains in the part of olivine structure.