Fibrous filter media are widely applied to remove aerosol particles. The modeling and prediction of filtration performance of fibrous media is of importance to design filter media targeted for specific applications. In this work, we successfully developed a 2-D numerical modeling for fibrous filter media. In our modeling, the flow field and fates of particles were calculated in model filter media having the fiber size distribution, average solidity, and thickness the same as those of real media. An excellent agreement was obtained between the numerical and experimental particle collection efficiencies of two commercially available fibrous filter media for particles in the sizes ranging from 3 to 500 nm. We also investigated the effect of fiber size polydispersity (both in the unimodal and bimodal fiber size distributions) on the particle collection efficiency of fibrous media. It was found that the particle capture in fibrous media is noticeably influenced by the polydispersity of fibers in a unimodal distribution, especially for particles in the sizes ranging from 10 to 100 nm, and further effected by the peak size and volume fraction in each mode of a bimodal fiber size distribution.