IASI is an infrared atmospheric sounding instrument devoted to the operational meteorology and to atmospheric studies and is to be installed on board the second ESA Polar Platform METOP-1 to be launched in 2001. The required operating lifetime is 4 years. An overall design of the Cold box, located at the focal plane interface level of the Michelson interferometer, has been performed for the mechanical structure and for the optical system taking into account the cold temperature requirement. This structure is interfaced with a passive cryogenic radiator. A thermal and modal analysis of this unit has been modeled to evaluate the thermal gradient and the eigen frequencies, by means of a finite-element software involving several thousands of nodes. Sinusoidal and random loads have been applied and the strains deduced. A thermoelastic model gives the relative position shift of the constituents when cooled-down. SAT is responsible for the subsystem Cold box and has been developing most of the critical elements of this unit. The role of this sub-assembly is to focus the IR signal onto the 3 detection arrays. It includes a spectral separation using 2 diochroic plates dividing the incoming flux into 3 spectral bands which are 3.6 to 5.0 micrometers , 5.0 to 8.26 micrometers and 8.26 to 15.5 micrometers . Each array stands behind an objective and a set of 4 microlenses. This unit defines the aperture and the field of view of the instrument and operates at 100 K with passive cooling. A specific optical ray tracer has delivered the optimization and the tolerancing of the optical system design. The results show that, with severe tolerances, the optical losses and cross-talk meet the requirements. The B-phase study is devoted to the preliminary design of the instrument and subsystems and to the manufacture of the critical components, among which the Cold box including all elements like IR detectors, optics, and packaging.