Electret filters have been used widely to collect particles because of their low pressure drop and high collection efficiency due to the electrostatic attraction between the particles and filter. However, the collection efficiency of these filters decreases over time due to charge degradation. Therefore, a unipolar charger using a corona discharge was combined with an electret filter to overcome this decrease. The highest particle collection efficiency for 70 nm, uncharged particles occurred with a fresh electret filter was 34.0%, whereas a neutralized filter had a collection efficiency of 32.6%. However, when fresh and neutralized electret filters were used with a unipolar charger (electret field: 1.9 kV cm–1), the highest particle collection efficiencies of 99.8% and 98.2%, respectively, were obtained and appeared at the electrode-to-mesh distance, a distance similar to the duct diameter. The distance between the tip of the discharge electrode and filter, which can be predicted using Sigmond’s law, affected the charge density value on the filter surface. The ozone concentration was negligible under optimum conditions for particle collection. Therefore, using an electret filter combined with a corona discharge has good potential for improving air purifiers.