The structure of metal fuel used in fast reactors is a complex function of composition, porosity, sodium content, texture, defect concentration, and crystalline phases. Of these, crystalline phases are arguably the most important in determining thermal, mechanical, and chemical behaviors of the fuel, and thus dominate the response of the fuel to a given set of operational conditions. This contribution will report on the crystalline phases observed in irradiated U-Pu-Zr fuels, which have been identified using selective area electron diffraction (SAED). In the past, phase identification in both fresh and irradiated fuels was done sporadically using less reliable methods. In this contribution, we report on the systematic radial microstructural examination of irradiated U-Pu-Zr fuels performed using SEM, focused ion beam (FIB) instrument, and transmission electron microscope (TEM). The crystalline phases in irradiated fuels were conclusively identified and the results compared to unirradiated fuels.