Abstract: Refractivity profiles observed by COSMIC(the constellation observing system for meteorology,ionosphere,and climate) GPS RO(radio occultation) mission were validated with global radiosonde soundings.The COSMIC RO events that occur within 2 h and 300 km of radiosonde soundings were gotten during the period from June 1 to June 30,2007 and more than 4 000 pairs of refractivity profiles were analyzed.The differences between the two types of refractivity profiles are the smallest in high latitude region and the largest in low lat-itude region.The same trend is also found in the comparisons between COSMIC RO and NCEP(National Centers for Environmental Prediction) refractivity profiles.We concluded that the precision of RO refractivity profiles is better in high latitude region than that of in low latitude region.There are systematic biases between RO and radiosonde refractivity pro-files in middle and low latitude region.No systematic bias can be found in the comparison be-tween the corresponding COSMIC RO and NCEP refractivity profiles.We deduced that the quality of radiosonde data could be the main reason for that bias.