Particulate matter has become the most important pollutant source with the rapid development of industrial economy. The aerosol particle number size distribution may be more important than the mass density in the view of its harm to human health. So the research on the aerosol particle number size distribution appears to be more and more important today. By using the fine particle meter developed by Anhui Institute of Optics and Fine Mechanics, an online, wide-range and quickly measurement of aerosol particle number size distribution in Beijing city during the Olympic Games was performed without making any changes to the samples. The variation trend of daily average and real time of particle matter number concentration, the particle number size distribution during the Olympic Games were analyzed. The results show that the particle number concentration of nucleation mode (5[EQUATION]20 nm) has close relationship with the nucleation process, and the concentration curve is unimodal. Particle number concentration of atiken mode (20[EQUATION]100 nm), with curve of three peak values, is strongly influenced by human activities and growing process of nucleation mode. Accumulation mode, with a tiny trend for its day average particle number concentration, is strongly affected by the weather conditions, especially in strong wind and heavy rain. The concentration of accumulation mode can directly influence the atmospheric visibility.