Laihunite, an apparent nonstoichiometric olivine-type mineral, shows a 3C type superstructure. This mineral has a basic cell of a=4.805(2)Å, b=10.189(9)Å, c=5.801(3)Å and α=91.0(2)° and the space group P2₁⁄b(a=unique axis) for the average structure. The basic structure has been studied by the single crystal X-ray method, starting from the model presented by Fu et al. (1979) with 889 reflections collected by four-circle diffractometry. The final weighted R and R factors are 0. 067 and 0. 075, respectively, after the three-dimensional least squares refinements. The mean T–O, M(1)–O, M(2)–O and V–O distances are 1.637(3), 2.185(1), 2.040(2) and 2.205(2)Å, respectively. The site occupancies and the bond distances of M(1), M(2) and V sites indicate the following distribution of iron atoms; V (Fe²⁺ 0.20), M(1) (Fe²⁺ and Mg²⁺, ∼1.00) and M(2) (Fe²⁺∼0.20, Fe³⁺∼0.80). The chemical formula of laihunite in this study is, therefore, approximately     _0.40Fe_0.80²⁺Fe_0.80³⁺SiO₄, indicating nonstoichiometry of laihunite.