This paper is concerned with theoretical and experimental investigations of the onset voltage of positive corona as influenced by coating the coronating electrodes. Dielectric-coated hemispherically-capped rod-plane gaps positioned in air are investigated. The onset voltage is determined when a developed primary avalanche triggers successor avalanches to sustain the discharge process. Laboratory measurements of the onset voltage on bare and coated electrodes were carried out. The effects of varying field nonuniformity, thickness and permittivity of coating material on the onset voltage values were investigated. The results show that coating the electrodes with a dielectric material is effective in increasing the onset voltage of corona. The calculated onset voltage values for coated and bare electrodes agreed satisfactorily with those measured experimentally within 10 %.