Abstract: Atmospheric particles with size range 17-800 nm were measured in urban area of Guangzhou from June 2 to July 15,3. The particulate matter (PM) pollution characteristics were investigated based on the data of number concentrations and size distributions of PM as well as hourly average concentrations of PM_2.5 and meteorological parameters. During the whole sampling period, the number concentrations of nucleation, aitken and accumulation modes ranged from 68 to 7,7, 1,9 to 47,4 and 238 to 14,1 cm^-3 respectively. The average number distribution was fitted with two lognormal distributions with geometric mean diameters at 48 nm and 144 nm. The average number and volume distributions exhibited peaks at ~50 nm and ~300 nm respectively. Particle number size distribution showed clear diurnal variation, with peaks related to traffic and strong solar radiation. Particle growth was frequently observed. Secondary photochemical formation contributed greatly to the PM pollution in Guangzhou. A typical pollution episode occurred from July 12 to July 13, during which the concentration of PM_2.5 sharply rose from 18 μg/m³ to 112 μg/m³, and visibility decreased to 8 km. PM_2.5 concentration was strongly correlated with the number concentrations of particles in accumulation mode. Back trajectory analysis showed that air masses originated from the southwest direction and stayed in the continent for a long time during this pollution episode.