The melting temperature of oxide fuel is generally considered to decrease with increasing of burnup because of the buildup of fission products. Melting temperatures were measured for UO2 and (U, Pu)O2 fuels. The (U, Pu)O2 specimens were irradiated to a burnup of 110 GWd/t in FBR. The solidus temperatures of (U, Pu)O2 were almost independent of burnup up to 50 GWd/t. The melting temperatures were correlated with Pu content, oxygen-to-metal ratio of the fuel and burnup by ideal solution theory.