In this study, the size distribution of atmospheric aerosol in Beijing was monitored by the scanning mobility particle sizer spectrometer and the optical particle sizer. The size of particles in atmospheric aerosol was primarily distributed in the range of less than 1 mu m. It showed different changes with the mass concentrations of particulate matters with an aerodynamic diameter of a (c) 1/22.5 mu m (PM2.5) for different sizes of fine particles. The amount of ultrafine particles (less than about 60 nm) decreased while the larger ones (> 60 nm) increased along with the mass concentration of PM2.5 in atmospheric aerosol. This was because of the formation of the secondary atmospheric aerosol. The polylactic acid (PLA) nanofibers were prepared for filtering the aerosol by electrospinning. PLA nanofiber mats were used as the middle layer to design the composite filter membranes. Atmospheric aerosol was used as dust source in the filtration test. The results showed that the filtration efficiency of the composite filter media increased along with the thickness of nanofiber mats, which was controlled by the collection time during electrospinning. Filtration efficiency can be improved obviously by compositing with a thin layer of nanofibers.