A scanning mobility particle sizer was utilized for the real-time measurement of particles in the size range of 14.6 nm to 661.2 nm from August 14 to November 18, 2014, to characterize the evolution of particle number size distribution and new particle formation (NPF) events in Lanzhou, China. The mean number concentrations of nucleation mode, Aitken mode, accumulation mode, and total particles were 2833, 12,898, 6210, and 21,940 cm(-3), respectively. The mean number size distribution was obviously unimodal, with a peak at around 60 nm. The diurnal variation of nucleation mode particles was mainly influenced by NPF events. Two peaks at traffic rush hours were observed for Aitken mode, accumulation mode, and total particles; the peaks were related to traffic and cooking soot emissions. The particle number and mass concentrations were all higher on workdays than on weekends mainly because of the enhancement in human activities and traffic density. Daytime also had higher values of number concentrations than nighttime. The mass concentrations of total submicron particles, sulfate, nitrate, and ammonium had much higher values at daytime than at nighttime, however, organics had slightly higher mass concentrations at nighttime mainly because of heating activities at night and intensified evening peaks. During the measurement period, the frequency of NPF events was calculated to be 34%. The average apparent formation and growth rates of nucleation mode particles were 1.71 cm(-3) s(-1) and 6.10 nm h(-1), respectively. A typical NPF event with two-stage particle growth was selected and analyzed to provide insight into the evolution of particle number size distributions. Back trajectory clustering analysis indicated that air masses that originated from the west had high values of both particle number and mass concentrations associated closely with local source emissions. Air masses from the north and northwest represented the relatively clean air mass arriving in Lanzhou.