For the control of particulate emission from diesel engines, particulate filter is of great importance. Fibrous filter is one of the most common filters due to its simplicity in structure and low cost in manufacturing. In this paper, filters with rectangular fibers are studied numerically, in which the filter is simulated as a staggered array of parallel rectangular fibers placed transverse to the flow. Two-dimensional flow field is obtained by solving the Navier-Stokes equations with the finite volume method Particle trajectories are then calculated by solving the corresponding Lagrangian equation of motion to obtain the collection efficiency of filters. The simulation considers particle capture mechanisms of interception and inertial impaction. The effects of fiber aspect ratio, filter packing density, particulate size on the collection efficiency of a rectangular fiber are also numerically determined.