Inversion of atmospheric Spectra measured by the satellite IASI instrument requires a code of radiative transfer coupled with an inversion procedure. We use the code of radiative transfer KOPRA (The Karlsruhe Optimized and accurate Radiative transfer Algorithm) and its inversion KOPRAFIT (http://www-imk.fzk.de/asf/ame/publications/kopra_docu/) module. This code was originally developed for the treatment of the MIPAS/Envisat at the IMK/KIT (Institute for Meteorology and Climate Research/Karlsruhe Institute of Technology). Used for the treatment of the IASI observations, we adapted the code to the measures performed in the nadir.