Particle number size distribution in Beijing was simulated during the period 15 January to 13 March 2006 using NAQPMS+APM (Nested Air Quality Prediction Modeling System with Advanced Particle Microphysics). The simulated number size distribution of fine particles agreed well with observations. The simulation results showed that new particle formation occurred frequently. The nucleation-mode particle number concentration increased drastically due to the nucleation process during new particle formation. In pollution periods, the number concentration of accumulation-mode particles increased, while the nucleation-mode particle number concentration deceased obviously. Moreover, the particle number size distribution shifted towards larger diameters. In heavy pollution episodes, the particle size increased significantly due to intense mixing through microphysical processes. The sizes of primary particles could be 50% larger than their core size due to secondary species coating on them. Accumulation-mode secondary and primary particles together contributed to pollution formation. For Beijing and surrounding areas, the simulation results showed that primary particles dominated particle numbers in the south of Beijing and Hebei, while secondary particles were dominant in the northern part of Hebei.