:IASI (Infrared Atmospheric Sounding Interferometer) is an Earth observation instrument carried by the METOP polar platform (launch foreseen in the beginning of the second millenary) in low earth (altitude of 838 km) heliosynchronous orbit with a local time descending node of 9.00 A.M. The programm is in phase B and under responsability of CNES and ASI (Italian Space Agency). Its mission is to measure the emission spectra of atmosphere in the field 3.4 to 15.5 μm. In this range of wave lenght, desired performances require cooling of the detection unit around 100 K.After a trade off, a cryogenic passive radiator has been preferred to a mechanical cooler for reliability, life duration, simplicity and absence of induced vibrations reasons. This paper describes the thermal concept of the radiator used to cool down the detectors. The detectors dissipation is 56 mW.The major problem in achieving cryogenic temperatures with passive radiator systems is to insulate thermally the system from the spacecraft and its external environment.In order to avoid direct solar input on radiative plates, a sunshield is used (cylindrical for manufacture aspect). Due to its form and its high level of specularity, the major part of albedo and earth fluxes coming on the sunshield are space rejected. Then, an earthshield is used to minimize the albedo and earth fluxes on the sunshield. Moreover, the radiative plates are in direction of Earth limb, so they can't receive direct albedo or earth fluxes.In order to reduce thermal inputs coming from the spacecraft, the cooler is a two stages cooler, each stage beeing thermally insulating from each other by multilayers insulation for radiative aspects and by low conductive support (glass fiber) for conductive aspects. This multistage concept reduces parasitic heat leakage to the cold stage.Each stage has its own radiative plate with high emittance to reject heat loads to space. The detectors are on the coldest stage.