Ag-based solder is an important type of solder based on silver or silver solid solution, which is being wildly used in various kinds of welding. Ag-based solder can be applied in welding of all the ferrous alloys and non-ferrous alloys expect Mg-based alloys, because of its low melting temperature and high qualities of wetting, filling in blanks, slot intensity, plasticity and so on. As Cu is one of the most important alloy elements in Ag-based solder, other elements are often added into the solder, such as Zn, Cd, Sn, Ge, in order to decrease the Ag-Cu solder melting temperature or substitute the expensive Ag. Among these additives, Cd could significantly low the liquidus of Ag-Cu-Zn alloy, and lessen the area between the liquidus and solidus at the same time. However, Cd is poisonous that it can only be forbidden.In the research of Cd-free solder, rare earth elements (RE) attract much. attention as they can refine the mechanical performance and wettability of Ag-based solder. Moreover, RE could reduce the oxidization of Ag-based solder and clean the grain boundary, such that improve the fatigue intensity. Development of Ag-Cu-RE solder is of much importance because of the abundant RE resources and its large markets. Using CALPHAD technique, Ag-Gd, Ag-Y and Cu-Gd binary systems are optimized and Ag-Cu-Gd, Ag-Cu-Y ternary systems are calculated,1. Ag-Gd, Ag-Y and Cu-Gd binary systems are optimized by Thermo-Calc software using CALPHAD technique. Liquid and all the terminal solutions (Fcc, Bcc and Hcp) are modeled as substitutional solution. All the intermetallic compounds are treated as stoichemistric phases. Calculated thermodynamic data and equilibria data agree with the experiments.2. Ag-Cu-Gd and Ag-Cu-Y ternary systems are calculated by Pandat software using CALPHAD technique. Solubility is added for some binary compounds and the ternary compound in Ag-Cu-Y system. AgY and CuY are treated as (Ag, Cu)Y because of their continuous solubility. Calculated phase diagrams are identical with the experimental data.