The effects of a carbon fiber ionizer on the collection efficiency of an electret filter for residential house ventilation were investigated with varying the face velocity, the number of ionizers, and the polarity of the ionizer. A Flimmer filter, which was made by a Swedish company, was used as the electret filter. The air ion concentration upstream of the filter gradually increased as the velocity increased. Under the same conditions of velocity and the same number of ionizers, the positive ion concentrations were slightly higher than the negative ones. The ion concentration at any location away from the ion source was only slightly affected by the number of ionizers. When ionizers were used in front of the Flimmer filter, the collection efficiencies for submicron particles increased by 10 to 25%. When positive ionizers were used, the collection efficiencies were 10 to 15% higher than with the negative ionizers. The increase in collection efficiency occurred because most of the generated air ions were captured on the surface of the electret filter fibers, thereby increasing the electrostatic potential inside the electret filter.