We have recently developed the theory of the dielectric constant of ionic crystals on the basis of the quantum theory of solids. In the present paper, we have worked out theoretically the temperature effect of the dielectric constant on the similar line as above, namely, we have determined the wave functions of ions so as to minimize the free energy change of the system under the electrostatic field, The numerical value of the coefficient of the temperature variation of dielectric constant of LiF crystal becomes of 3.2×10-4 in comparison with the observed value of 3.75×10-4.