Through ELPI (Electrical Low-Pressure Impactor) and dilution sampling system, the mass size distribution, enrichment mechanism, and emission factor of trace elements in PM10 was studied experimentally at the inlet and outlet of electrostatic precipitator (ESP) at two pulverized coal-fired boilers. Results showed that a bimodal mass size distribution of trace elements occurred, similar as that of PM10. It was presumed that the enrichment of trace elements in submicron mode should be controlled by heterogeneous chemical reaction, while the enrichment in supermicron mode by either chemical reaction or heterogeneous coagulation. According to the relative enrichment factors calculated, all studied elements showed enrichment trend in submicron mode except that Mn showed exhaustion trend in PM10. Since the removal efficiency of ESP is low in the particulate size range of 0.1-1 μm, the removal efficiency of most trace elements is lower than that of particulate in PM1, PM2.5, and PM10. Although the emission factors of trace elements varied much after ESP, the total emission of trace elements may be large and the environmental impacts should be much concerned.