This paper focuses on the modelling of fission gas release in mixed oxide fuel. In a first part, the irradiation experiment, conducted by the Japanese Nuclear Energy Safety organization on high enriched mixed oxide fuel, is outlined. In a second part, the approach for fission gas release modelling, as implemented in the fuel performance code MACROS, is explained and a comparison between calculated and experimental results is made. The code MACROS is conceived to provide not only integral (rod average) results on fission product and fission gas retention and release, but also to calculate local concentrations (radial profiles). In this way, it is possible to compare results from post-irradiation examinations with calculated profiles.