Previous studies carried out on the U-Nd-O ternary system highlighted the presence of the miscibility gap at room temperature when the concentration of Nd is higher than 5 at%. This biphasic region is characterized by two face-centered cubic (FCC) phases in thermodynamic equilibrium. The objective of the current work is to analyze the evolution of the crystalline structure in temperature and the presence of dislocations at room temperature as a consequence of phase separation. Three different samples with 4, 17 and 25 at% of Nd were studied through means of High Temperature-X-Ray Diffraction (HT-XRD), High-Temperature Scanning Electron Microscopy (HT-SEM), and Transmission Electron Microscopy (TEM). Experimental results are confronted to thermodynamic modelling calculated through the CALPHAD method using the Thermo-Calc code. In this paper a first approach on the evolution of the microstructure as a function of the concentration of Nd is also proposed.