The average value of g factor (i.e., g ) of Cr4+ ions in forsterite (Mg2SiO4) is calculated with the cubic symmetry approximation from the complete high-order perturbation formula of g factor for 3d2 ion in cubic tetrahedral cluster. In the formula, the contribution to g factor from the charge-transfer mechanism (which is neglected in the crystal-field theory) is considered in addition to that from the widely used crystal-field mechanism. From the calculations, the reasonable observed value of g is suggested (note: the experimental values of g by various authors are scattered) and the important contribution of charge-transfer mechanism to g factor can be found.