A model for constituent migration behavior in U-Pu-Zr metallic fast reactor fuel is proposed. It is based on diffusion equations for the ternary system under a radial temperature gradient, and it takes into account the alloy phase decomposition, assuming a quasi-binary U-Zr phase diagram with a constant plutonium content. Parametric simulations of Experimental Breeder Reactor II irradiation data with appropriate transport properties of the alloy system showed that the model can predict the experimentally observed radial three-zone structure and zirconium and uranium redistribution, although the predicted radial location of zirconium-depleted middle zone disagreed with the experimental result. Accumulation of basic experimental data on transport properties and a ternary phase diagram of the system are needed for a better understanding of the behavior.