The goal of experimental investigations shown in this paper is to estimate operating efficiency degree of the electrostatic precipitator on a real industrial plant (at thermal power plant ?Gacko? with electric power of 310MW, Bosnia & Herzegovina) and to use obtained results as a base of periodical engineering or continual measurement and to compare them with the investigations of other investigators. Investigation of the electrostatic precipitator performance was done according to . BASISO9096:2003 In this paper the electrostatic precipitator efficiency during ash particle removal with wide range of particle sizes from 1 to 250mm .93%is evaluated. Exploitational experience points that electrostatic precipitators are efficient for the coals of different quality (coal particles diameters bigger than 1) and that they could be optimized during the exploitation itself and for some following processes (e.g. flue gas desulphurization). Within the measurement plane, measurements were made on points per section. It has been noticed that ash removal degrees obtained experimentally (3 investigations) have approximately equal value (95 to 97.78). The best concordance with the results of experimental investigation shows the Deutsch equation, while theoretical models of Zhibin-Guoquan and Nobrega-Falaguasta-Coury do not correspondent well with the results of experimental investigations. For the ash particles with the diameters less than 17.5 there is no good correlation between investigated theoretical models. The highest deviation of the model for ash particles with diameters less than is notable in the case of usage of the Deutsch equation. ?m17.5?m20%?m.