An analysis is made of the dielectric behavior and the anharmonic contribution to the dialectic constants of alkali halides. The volume and temperature derivatives of static and electronic dielectric constants, and of the energy gap between bonding and anti bonding states are derived from the experimental data using Phillips Van –Vechten dielectric theory. The temperature derivatives of dielectric constants at constant volumes are used to estimate the magnitude of an harmonic contributions. The effect of an harmonicity is discussed in terms of different theories .The volume derivatives of static dielectric constant are3 calculated by using the Born –Mayer theory of interionic potentials. The calculated quantities are compared with available experimental data. .The applicability of the Mott Littleton theory of dielectric polarization is also discussed for the crystals under study.