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High-performance air filters composed of a hybrid structure of nanofiber/microfiber were fabricated using wet paper processing. Two types of nanofibers (NF) with average diameters of 180 and 234 nm were mixed with a suspension of microfibers (11.5 and 11.7 µm) in various mixing fractions. Then, the suspension was filtered to fabricate hybridized fiber sheets with a known nanofiber/microfiber composition. The effects of NF diameter and mixing fraction on the performance of the hybrid filters were experimentally investigated. With increasing NF fraction, both the particle collection efficiency and the pressure drop increased. The quality factor (Qf) was used to evaluate the performance of the prepared filters. As predicted by the single fiber filtration theory, the experimentally obtained Qf was almost independent of the mixing fraction of the NF. The collection efficiency and pressure drop of the hybrid filters could be controlled by the NF fraction at the same Qf. Moreover, the inhomogeneity factor of fiber packing (δ) did not significantly affect Qf over the δ range from 3 to 23 for our filters. This implies that the lower particle capturing efficiency due to heterogeneous packing could be compensated by a decrease in the pressure drop, resulting in the same Qf value. Therefore, Qf for particles smaller than 100 nm, which are in the diffusion-controlled regime, can be increased by reducing the NF diameter. Copyright © 2019 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]