12 Global Change Observation Mission (GCOM) is a follow on mission of ADEOS and ADEOS2. It is under phase A study in NASDA (National Space Development Agency of Japan). GCOM is not a series of satellites but a mission and its concept is to continuously monitor geophysical parameters which are critical to understand global change phenomena, especially phenomena related to climate change. The measurements of geophysical parameters will continue more than 15 years after the launch of ADEOS2. The first generation of GCOM is now composed of 2 satellites, i.e. GCOM-A1 and GCOM-B1. The target of GCOM-A1 is to monitor greenhouse gases distribution and ozone as well as ozone related constituents from oblique orbit. The target of GCOM-B1 is to measure geophysical parameters which are uncertain in today's climate models. Those parameters include, but not limited to, optical thickness of aerosols and clouds, thermal fluxes, carbon fluxes, sink and source of greenhouse gases, etc. GCOM-B1 will carry three core instruments, i.e. SGLI (GLI follow on), AMSR2 (AMSR follow on), alpha-Scat (SeaWinds follow on). Another candidate instrument is ATRAS (IMG follow on). The orbit of GCOM-B1 will be a sun synchronous orbit, which is almost the same as ADEOS2. GCOM- B1 is especially powerful for the monitoring of oceanic processes. It can observe sea surface temperature, ocean color and sea surface winds simultaneously by 3 sensors, i.e. SGLI, alpha-Scat and AMSR2. GCOM-A1 is planned to be launched in Feb. 2005 while GCOM-B1 is planned to be launched in Aug. 2006.