The filtration behaviors of glass filters were studied by measuring the air permeability, pressure drop and SEM images of five HEPA grade filter material. Based on the observations, the filtration mechanism was described in terms of fiber diameter, packing density and filter depth. The results indicate that the finer fibers and fibers on/near the filter media surface show better filtration performance. Also the captured particles on the fibers formed dendrites, which influenced further filtration.