By using the single particle model, the diamagnetic Faraday rotation (FR)spectra of Pr3+ ion in Pr:YIG crystal is presented based on the quantum theory. The linear dependence of λF-1/2θ on λ2 is calculated at 300 and 77K in the 105—13 micron wavelength range. It is found that the diamagn etic FR is 64% of paramagnetic FR at 77K and the two types of FR can be of more or less equal importance. The diamagnetic FR depends on the temperature and the exchange field for the ions with orbit single ground states derived from the cr ystal field.