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The physical properties and irradiation behavior of Np-and Am-bearing MOX were evaluated for the development of advanced fast reactor fuels. The physical properties, lattice parameter, melting temperature, thermal conductivity, and oxygen potential of the fuels were described as functions of Np content, Am content, and oxygen-to-metal (O=M) ratio, and the effect of a few percent Np and Am addition into MOX on those properties was found to be negligibly small. The irradiation tests of fuel pellets having O=M ratios of 1.98 or 1.96 were carried out at a high linear heat rate of about 430 W/cm for 24 h. The redistributions of actinide element and oxygen were analyzed by using the physical properties for Np-and Am-bearing MOX, and the maximum temperatures of the pellets were estimated. The maximum temperatures of the pellets of O=M ¼ 1:96 and 1.98 were estimated to be 2,701 and 2,510 K, respectively. The analysis result showed that the higher maximum temperature in the low-O=M pellet was caused by the low thermal conductivity in the outer region of the pellets, and the maximum temperatures of the pellets did not exceed their melting temperatures. In the results of postirradiation examination, no trace of melting was observed.