The results of theoretical and practical study of a microwave multipactor discharge in the round cylindrical cavity resonator for diagnosing of high-voltage insulators technical condition are presented. The analysis of the resonator main characteristics, which determine the condition for the occurrence of a multipactor discharge, taking into account the geometric dimensions and electrophysical properties of the resonator, is given. On the example of the porcelain insulator as a dielectric material, the peculiarities of the interaction mechanism of the standing wave in a resonator with the surface of the diagnosed product placed in it are revealed. The calculation of the microwave radiation absorption coefficient by a multipactor discharge on a dielectric plate is given as the main information parameter of the insulator defectiveness degree. Experiments have established the possibility of the insulator state diagnosing by means of a multipactor in the absence of external static fields.