In order to research the effect of high humidity on the growth of aerosol particles, the APS, TEOM and BC instruments were used to monitor the aerosol in Beijing during August, 2007. The results show that, under steady-state weather condition, the correlation coefficient between the mass concentration of PM10 and BC aerosol is 0.82, but the correlation between the number concentration and the mass concentration of PM10 is not in accordance with that under non-steady-state weather condition. Tie number concentration and the ratio of fine particles less than 1 microm to the whole samples increase obviously. The change of aerosol particles size distribution suggests that the size of aerosol particles increases obviously by moisture absorption under the high humid condition.