High performance Nd-Fe-B permanent magnets have been used in many industries, such as wind power, electric vehicle and hard disk [1].Better temperature stability, higher energy product and coercivity of the Nd2Fe14B-based magnets with heavy rare earth elements such as Dy, Tb could be achieved [2].To decrease the usage of heavy rare earth elements (e.g.Dy, Tb) without sacrificing the magnetic performance [3], light rare earth are always been added to the Nd-Fe-B permanent magnets to form the two main phases structure [4].For further understanding of how the two main phases structure can perfom such a better performance than traditional Nd-Fe-B permanent magnets and to make full use of light rare-earth-elements,the investigation on the phase relations and phase diagrams of Fe-RE systems are needed.In this work, the Fe-RE binary systems were firstly investigated experimentally by means of differential thermal analysis (DTA), and then were thermodynamically assessed based on critical review of the reported experimental phase equilibria data and thermodynamic data using the CALPHAD method through Thermo-Calc(R) software package [5].The self-consistent thermodynamic parameters obtained finally to describe the Gibbs energies of various phases in the Fe-RE binary systems can be used to reproduce well the reported phase equilibria and thermodynamic data.