An electrostatic precipitator is an air cleaner in which particles are first charged by an electric current coming from corona wires in high voltage. Then the particles are collected by the electric field between collecting plates. The principle of the operation of an electrostatic precipitator is shown in Fig. 1. An important property of the electrostatic precipitator is the filtration efficiency. For high efficiency the charging current and collecting voltage must be sufficiently high. The higher the current the more ozone is produced. The high collection voltage may also cause ozone- producing discharges between collection plates. The aim of this study was to investigate the filtration efficiency and ozone-production dependence on charging current and collecting voltage. This knowledge can be used when electrostatic precipitators for dwellings and public rooms are designed.