Abstract: The COSMIC (Constellation Observing System for Meteorology Ionosphere and Climate) consists of six Low Earth Orbit (LEO) satellites. Based on the GPS signals observed by LEO in the occultation condition of the GPS satellites, the profiles of atmospheric temperature, pressure, humidity and ionosphere are retrieved. COSMIC provides these to about 2000 to 3000 GPS soundings globally every day since September 2006, and it can be the effective supplementary of regular radiosonde. COSMIC data is collected in the area of China and Northwest Pacific during the period of January to October in 2007.And the precision, applicability and characteristics of fitting all weather condition are evaluated by the error statistics based on radiosonde data. The results show that the accuracy of temperature profiles retrieved from COSMIC is excellent. The root-mean-square error (RMS) is less than 2 K and the relative error is less than 1%. The retrieved refractivity above 500 hPa level is very accurate, with the RM S less than 1, while under 500 hPa level the RMS increases from 1 to 10 near the ground. The precision of water vapor is high above 500 hPa level, but the RMS increases linearly from 0.2 hPa or less to 2 hPa near the ground under 500 hPa level. Meanwhile the error comparison between COSMIC data and NCEP (National Center of Environment Precipitation) reanalysis data shows that COSMIC data have higher accuracy than NC EP in vapor pressure and refractivity, for instance, the relative error of vapor pressure of COSMIC is 2%-5% less than NCEP data. Additionally two COSMIC occultation events in the clouds are studied to compare COSMIC data with the nearby radiosonde and airborne dropsonde observations. It' s validated that the profiles of the temperature, vapor pressure and refractivity of COSMIC data coincide well with the corresponding radiosonde profiles, so the COSMIC data are appropriate under all weather condition.