The FORMOSAT-3/COSMIC spacecraft constellation consisting of six LEO satellites is the world's first operational GPS radio occultation mission. The mission is jointly developed by Taiwan's National Space Organization (NSPO) and United States' UCAR in collaboration with NASA's Jet Propulsion Laboratory and Naval Research Laboratory for three onboard payloads including GPS Occultation Receiver (GOX), Tri-Band Beacon (TBB), and Tiny Ionospheric Photometer (TIP). The FORMOSAT-3/COSMIC mission was launched successfully from Vandenberg on April 15, 2006 into the same orbit plane of the designated 516 km circular parking orbit altitude. All six FORMOSAT-3/COSMIC satellites are maintained in the good state of health except spacecraft flight model no. 2 (FM2) having power shortage and are on their way toward the final constellation of six separate orbit planes with 30-degree separations as planned. Four out of six satellites have reached their final mission orbit of 800 km as of May 2007. The FORMOSAT-3/COSMIC has processed over 1800 good atmospheric sounding profiles per day in average which has over the number of worldwide radiosondes launched per day (∼900 mostly above the land mass). The atmospheric radio soundings data are assimilated into the Numerical Weather Prediction (NWP) models for real-time weather prediction and typhoon/hurricane forecast. The global and nation's weather prediction centers have shown significant positive impact and the forecast result will be also adapted into the nation's disaster warning and relief system once constellation deployment completed by end of year 2007. With the invention of the open loop technique by UCAR, the quality, the accuracy and the lowest penetration altitude of the RO sounding profiles are better than CHAMP data. This paper also describes the mission highlight; the constellation spacecraft system performance summary; the constellation mission operation result; and the mission science results.