Phase diagram of Mg-Sn-Y ternary alloy systems were constructed thermodynamically using calculation of phase diagram (CALPHAD) approach. The thermodynamically calculated isothermal section of Mg-Sn-Y ternary alloy at 400 °C was validated by comparing with results reported in the literature and few selected key experiments. The calculated results matched well with the experimental results. Vertical section phase diagram of Mg-1Sn-xY and Mg-3Sn-xY alloys were constructed on the basis of solidification analysis and compared with thermodynamically calculated phase diagram. The thermodynamically calculated liquidus temperature for Mg-1Sn-xY system matched well with the experimental results. However, significant differences in solidus temperature and solubility limit were observed for Mg-1Sn-xY alloys. This difference is attributed to the shift in composition and temperature for L→L+α-Mg + Sn3Y5and L→α-Mg + Sn3Y5+Mg24Y5phase transformations. In Mg-3Sn-xY phase diagram, a gradual decrease in liquidus temperature is experimentally observed up to 13 wt % Y and further addition of Y leads to increase in liquidus temperature. The increase in liquidus temperature is attributed to the formation of MgSnY intermetallic compound which is not predicted by thermodynamic model. The lack of experimental data on the enthalpy of formation of ternary MgSnY compound is one of the major reasons for the discrepancy between the thermodynamic calculation and experimental results. © 2016 Elsevier B.V.