Abstract: The number concentration and size distribution of ultrafine particles(UFPs) in urban areas and suburban industrial areas of Hangzhou City,Zhejiang Province were tested using FMPS(fast mobility particle sizer).The results showed that in the range of 5 to 500 nm,the atmospheric particles mainly showed a logarithmic bi-modal distribution,with most CMD(count median diameter) from 10 to 50 nm.The highest average number concentration was 4.11×104 cm-3,found in the suburban industrial areas,and the peak concentration was detected at noon.The UFPs pollution in the urban areas was relatively light.The diurnal variation of the number concentration of UFPs illustrated that it was high during rush hours and generally lower at night,although fluctuations occurred occasionally.Some factories worked overnight,so sufficient attention should still be paid to the air quality at night.