Electret filters are frequently used in air cleaners because they have a lower pressure drop than the mechanical filters at the same collection efficiency. Manufacturers of electret filters have tried to increase the electrical charge density of electret fibers in order to improve the collection performance. Through these efforts, high-performance electret filter (HPEF) is developed and a question is raised on whether the previous prediction equations for collection efficiency are applicable to these HPEF because the prediction equations were never tested for such high charge density electret filters. In the present work, we measured the collection efficiencies of recently-developed HPEF and studied the applicability of previous prediction equations for collection efficiencies. As a result, HPEF had the electrical charge density twenty times as high as that of the previously-studied electret filter and the single fiber efficiency was close to the maximum limit determined by the packing density of fibers. Furthermore, it is found that the electrical charge of HPEF is fairly stable against organic solvent, leaving one-third of initial charge after soaking it with ethanol. In predicting the collection efficiency of HPEF, the conventional equation can be applied to the collection by induced force, however that by Coulombic force is not applicable because the single fiber efficiency due to induced force and Brownian diffusion is high and close to the upper limit without the collection by Coulombic force.