Fibrous filter, being simple in structure and low cost in material, is widely used in fine particulate removal. The collection efficiency of a fibrous filter is heavily dependent on the collection efficiency of a single fiber. Despite the availability of a reasonable formulation for single fiber collection by diffusion, most expressions of collection efficiencies by interception and impaction are empirically based and few simple analytical formulations are available, especially for impaction-dominated collection. In this paper, impaction-dominated fibrous filtration with rectangular fibers for particulate size much smaller than that of the collecting fibers are both numerically and analytically investigated. The effects of fiber aspect ratio, filter packing density, particulate size and Stokes number on the collection efficiency of a rectangular fiber in the impaction-dominated filtration are numerically determined. In addition, simple analytical expressions of single fiber collection efficiencies due to interception and inertial impaction of fine powders are derived based on Kuwabara flow analysis around a cylinder. Hence, using the equivalent hydraulic diameter concept for rectangular fibers with moderate fiber aspect ratios, the effect of particulate polydispersion on the filter collection efficiency is investigated based on presumed mass-based Gaussian size distribution and log-normal size distribution, respectively. Differences between overall size-averaged collection efficiency and mean size collection efficiency can, thus, be estimated.