Abstract: A field measurement campaign was conducted in Dunhuang and Qinghai Lake sites in July 1999. Three CIMEL sun-photometers were used to measure the optical characteristics of the atmosphere at the two sites, and a great deal of data about atmospheric optical characteristics in the clear condition were obtained. Data from aerosol-bands of sun-photometers are used, by means of the Langley method, to determine aerosol spectral optical depths. The results show that the aerosol optical depth (AOD) at 550 nm at the two sites is 0.125 and 0.18 respectively. ν (Junge aerosol size distribution parameter) derived from spectral variation of AOD is 2.6 and 3.0 respectively. These values are compared with several typical aerosol models. All results show that the aerosol concentration at the two sites is small and suitable for in-flight calibration for remote sensing satellite sensors.