Using crystal field (CF) theory and ab initio MO calculations, the g-tensor components of the [Ti(H2O)(6)](3+) complex have been computed in order to elucidate the reason that the [Ti(H2O)(6)](3+) complex has a large g-factor anisotropy in the alum crystal (g(parallel to) = 1.25 and g(perpendicular to) = 1.14). Ab initio MO calculations show that the H-O-H plane is parallel to the C-3 molecular axis at the most stable paint, suggesting that the complex has D-3d symmetry. The g-tensor components of the complex with the D3d symmetry are in good agreement with experimental results obtained in the amorphous solid (g(parallel to) = 2.00 and g(perpendicular to) = 1.86). A simple model in which the H-O-H plane is rotated from the C-3 axis is considered to explain the large anisotropy in the alum crystal. It is found that the rotation of the H-O-H plane relative to the C-3 axis causes both the energy shift of low-lying doublet states and the large anisotropy of the g-tensor components. This is due to the fact that the repulsive interaction between the d-orbital and the nonbonding orbital of the ligand H2O molecule causes the energy shift. The g-tensor components calculated for the rotated structure are in reasonable agreement with those in the alum crystal. It is concluded that the rotation of the H-O-H plane relative to the C-3 axis is the origin of the large anisotropy of the g-factor in the alum crystal.