A technique for validating General Circulation Models has been developed in the framework of the AMIP subproject No. 10. This technique is based on the diagnostics of the process of the atmospheric blocking. It computes frequencies of blocking occurrence as function of longitude and seasonal cycle, as well as blocking duration distribution and other statistical and synoptic quantities. The diagnostic package is based on an objective blocking index whose validity has been proved in previous works. Only a part of the outputs of the models participating to AMIP has been analysed, and the preliminary results will be presented here. Large differences in the ability of reproducing blocking are found among the different models. Some interpretation of these differences is attempted.