Abstract: The humidity-controlled aerosol measurement system was developed to study the hygroscopic properties of aerosols. The system included a humidity-controlled inlet to humidify the sample air, and two nephelometers to simultaneously measure the aerosol scattering coefficients under “dry” and “wet’ conditions. During the measurement, the air passed through the inlet and was humidified by the water vapor diffusing through the wall of the membrane tube. The method used to adjust the amount of water vapor, was called the “combination of water vapor addition and/or thermal control” method, was realized by controlling the temperature of water out of the membrane tube. Using this system, the experimental study on the hygroscopic growth of aerosol scattering coefficients at an urban site (CAMS) in Beijing city was conducted from December 7 to 22, 2005. The instrument was installed on the roof of a building on the campus of Chinese Meteorological Administration in the northwest of Beijing urban area, and the inlet of the instrument was about 50 m above the ground. The preliminary analysis of the measurements indicate that: overall, the hygroscopic growth factor of the scattering coefficient f(RH) increased continuously when the RH increased. the average growth factor f(RH) of the aerosols for the whole measurement period could reach to 2.1 when RH increased from less than 40% to 93％. The average hygroscopic growth factor at relative humidity of 80%, f(RH=80％±1%), was about 1.26 ±0.15. Further calculation shows that the hydroscopic growth factor f(RH) was relatively higher when the air was relatively polluted, where, the f(RH) at RH=80% was about 1.48, however, when the air was clean, the growth factor f(RH) at RH=80% was about 1.2. The growth factor f(RH) for relatively polluted case was lower than the result reported by Carrico (Carrico et al.,2003), f(RH=82%)＝2.22±0.20, for polluted aerosol type during the Ace-Asia experiment, but the behavior of the hydroscopic growth under clean air conditions was similar to those influenced by burning biomass or blowing dust as reported (Carrico et al., 2003; Kotchenruther et al, 1998). These results reflect the different characteristics of aerosol types at Beijing urban area under the different air conditions in the winter.