Size-resolved aerosol number concentrations (10 nm–10 µm in diameter) were measured at an urban site in Beijing during summertime of 2008. Case studies of new particle formation (NPF) are presented in this work. The measured mean particle formation rate was 2.37 cm–3 s–1, which varied from 1.5 to 3.8 cm–3 s–1, with growth rates ranging from 3.2 to 10.6 nm h–1. NPF was observed under low number concentration of preexisting particles as well as under relatively high number concentration of preexisting particles. It was found that condensation contributed mainly and preferentially to particles growth, however, coagulation would contribute a lot when formation rate of new particles was sufficiently high. The variation of concentration of nucleation mode particles was found to be coincident with sulfur dioxide, indicating that NPF could occur under relatively high number concentration of preexisting particles if sufficient concentration of gas-phase H2SO4 existed in the atmosphere. Grown particles were also observed to shrink from 61.1 nm to 15.4 nm at a shrinkage rate of 16.6 nm h–1, accompanied by a reduction of the particle number concentration. The shrinkage rate was higher than those reported in recent studies, probably due to particle shrinkage occurred during summertime in Beijing with higher temperature and lower RH compared to those observed in other regions, thus enhanced particle shrinkage.