Abstract The radial distribution of retained xenon concentration in the mixed oxide fuel pellets irradiated in JOYO MK-1 and Rapsodie up to a burnup of 100 GWd/t were measured using electron probe microanalyser (EPMA) and ion probe microanalyser (IMA). For the irradiated fuel up to a burnup level of 50 GWd/t at a linear power less than 300 W/cm, the local release of xenon occurred at the densified region just outside the equiaxed grain region in which fission gas almost escaped from the pellet. Beyond 50 GWd/t under around 450 W/cm, xenon release was accompanied by submicron size intragranular bubble formation just inside the unrestructured region. In the unrestructured region, the retained fission gas was found to be released at the burnups beyond 50 GWd/t, which is related to the saturation concentration of fission gas within grains. The fission gas release rates in unrestructured and restructured regions were correlated with burnup up to 100 GWd/t.