Abstract: The varistors are materials which belong to the class of ceramics and which in the beginning was primarily made up of Silicon carbide (Sic), but for a few years one has advantageously used the varistors Containing Zinc Oxides (ZnO); these last are used more and more for the protection of the electric components against overpressures.The varistors used as element of lightning protector on the lines of distribution of energy are subjected in a permanent way to the various types of constraints. We thus propose a study of the ageing of the varistors under the thermal stresses and electric. However, we examined, during the operation of ageing revolution according to the time of three sizes characterizing a varistor: the leakage current, the tension of threshold and the coefficient of linearity.