The phenomenon of back corona is well known in that it may reduce the collection efficiency of an electrostatic precipitator. In this study, experimental results are presented for the influence of back corona on the performance of a laboratory scale single-discharge-wire ESP system. The effect of back corona under two power controlled methods, constant voltage operation and constant current operation, has been evaluated. The experimental results showed that as back corona occurs, the output current and the power consumption are increased under constant voltage operation. The mass collection efficiency is proportional to the applied voltage. On the other hand, for constant current operation the voltage and the power consumption are decreased under back corona. The mass collection efficiency is independent of the applied current. A lateral mixing model was also applied to assist in the discussion of corona power and particle collection efficiency under normal and stable back corona conditions. Both experimental and theoretical results indicated that the back corona has a large effect on submicron particles. The grade efficiency can be reduced by over 60% for submicron particles. The large reduction of efficiency for submicron particles is due to significant reduction in both particle charge and migration velocity. Since particles in the submicron region are more sensitive to light scattering, the opacity problem may be aggravated under back corona.