This study reports the impacts of two commercial wet electrostatic precipitators (WESPs) on the emission of fine particulate matter (PM2.5) and SO3. Field measurements were carried out at two 300 MW coal-fired power station units equipped with WESPs between limestone-gypsum wet flue gas desulfurization (WFGD) unit and the stack. PM samples were collected at the inlet and outlet of the WESPs as well as the inlet and outlet of the dry ESP, and SO3 was collected at the inlet and outlet of the WESP. The results show that 99.21% of the PM2.5 emitted from the boiler is removed in the dry ESP. The WFGD has a removal efficiency of 24.19% for PM2.5 but produces new gypsum particles and then increases the emission of PM1 (by similar to 24%). PM at the WESP inlet is centered at 1 mu m, and after passing through the WESP, concentrations of PM0.3, PM1, and PM2.5 are reduced by 73.72-93.75%, 83.33-94.41%, and 79.91-90.23%, respectively. SO3 emission is also reduced by the WESP due to the capture of H2SO4 via electrostatic force and the absorption of SO3 by the basic sprayed liquid, with a removal efficiency of 52.03-59.09%. However, new PM larger than 2 mu m is generated from the entrainment of circulating liquid droplets in the WESP, which partially offsets the capture of PM2.5. As the boiler load decreases from 100% to 70%, the removal efficiency of PM0.3 increases a little, whereas the removal efficiencies of PM1 and PM2.5 decrease from 89.34% and 89.85% to 83.33% and 79.71% due to the decreased removal efficiency for PM larger than 2 mu m. WESP contributes less to the PM2.5 removal compared to the dry ESP (0.54% vs 99.27%) while it effectively reduces fine PM emission.