Abstract: Micron wood fiber pits have a strong adsorption to ultrafine particles exhausted from diesel vehicles with the kinetic diameter ranging from 0.01 μm to 1 μm, which can effectively reduce emissions of diesel vehicles PM_2.5. Making use of equations of particle concentration as well as the fiber collection theory, collection media around the pits were supposed into cylinders, and numerical analysis was carried out on the steady collection efficiency of micron wood fiber pits. The results show that micron wood fibers have a low collection efficiency to particles with the diameter from 0.4 μm to 0.6 μm, while they have a much higher collection efficiency to those particles in other sizes; among those particles with the diameter of 0.01 μm the collection efficiency of micron wood fibers can reach up to 90%, who plays a significant role on collecting ultrafine particles of diesel vehicles.Among all the factors on collection efficiency, reduction of surface seepage velocity and increase of the tracheid wall thickness would help improving the filtration efficiency, while the exhaust gas temperature has little effect on collection efficiency, which can totally be ignored.