The characterization and high temperature behaviour of U–15 wt%Pu alloy has been investigated in this study for the first time. U–15 wt%Pu alloy sample for this study was prepared by following melting and casting route. Microstructural characterization of the alloy was carried out by XRD and optical microscopy. The thermophysical properties like phase transition temperatures, coefficient of thermal expansion and hot hardness of the above alloy were determined. Eutectic temperature between T91 and U– 15 wt%Pu was established. Apart from that, the fuel–cladding chemical compatibility of U–15 wt% Pu alloy with T91 grade steel was studied by diffusion couple experiment.