Semi industrial-scale, two-stage electrostatic precipitation system comprised of unipolar electrostatic agglomerator and electrostatic precipitator was investigated in this paper. In this type of agglomerator, the process of particle charging and their agglomeration is accomplished in the same device. The particles are charged by ion current in alternating electric field, and agglomerated due to their oscillatory motion in this field, perpendicular to the gas flow. The charged and agglomerated particles are collected in the next stage, which is a conventional electrostatic precipitator with spiked wire discharge electrodes and collection electrodes of the sigma type. Collection efficiency of this system was measured for different gas temperatures, different fly ash concentrations and for various magnitudes of AC voltage applied to the aggiomerator. Two-stage electrostatic precipitator allows obtaining higher fractional collection efficiency for PM1 and PM2.5 particles than a one -stage electrostatic precipitator. In this type of two-stage electrostatic precipitator with an agglomerator, the number collection efficiency for fly ash particles in PM10, PM2.5 and PM1 size ranges was about 96%, 96% and 94%, respectively, and mass collection efficiency in same size ranges was 98%, 97% and 95%, respectively. (C) 2017 Elsevier B.V. All rights reserved.