The exchange interaction between electrons exists in the paramagnetic media CeF3. The relation of the effective exchange field Hin to the temperature is shown to be Hin=(-0.68-0.002T)×10-6M. By means of this relation and the quantum theory, the inverses of the magnetic susceptibility and Verdet constant of CeF3 crystal as a function of temperature are calculated. The calculated V-1-T and χ-1m-T curves are in good agreement with the measured results. The results are further discussed in Terms of the concepts and rules of photo-magnetic effect.