The main focus of this paper is on the study of the radiation effect upon the electrical property of a A-Kind insulator which is one of the organic insulators. The varnished cloth is so typical of the A-Kind insulators as to be selected for a sample. AC and Impluse voltage is applied to the variable Gamma ray irradiated dose samples with the constant time duration and the dielectric breakdown voltage are measured from the sample. The experimental data of the dielectric breakdown voltage are considerably decreased. The above conclusion is useful for the selection and application of the orgainc insulators under the irradiation effects.