This paper presents the results of an experimental investigation of corona discharge in Wire-to-Plane Electrostatic Precipitator (WP-ESP). The influence of relative humidity level (RH) on the collection efficiency of a WP-ESP is analyzed. Experimental investigations of the humidity effect on corona discharge in electrostatic precipitator are of interest for the interpretation of electrical conduction and collection efficiency phenomena in high electric fields. The experiments were performed with incense smoke particles having a mean size of about 0.28 mu m. An aerosol spectrometer was employed for characterizing the size distribution of these particles at the outlet of the ESP. Then, the collection efficiencies were measured for relative humidity levels of 40 and 70%. Various DC applied voltages in the case of positive and negative polarities have been under consideration. For given atmospheric conditions, the results obtained with the aerosol spectrometer show that the performance of the ESP increases with the applied voltage (range: 10-40 kV). Also, the collection efficiency is higher at increased RH. The negative corona discharge is overall more effective than the positive one. However, the difference between the two polarities becomes minor at high RH. The performances of the ESP are improved at elevated humidity probably due to the enhancement of the particle charging in the inter-electrode gap on one hand, and the particle cohesivity on the collecting electrodes on the other hand. (C) 2016 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.