Mg-Zn-Zr series Mg alloys (ZK) are one of the most important commercial Mg alloys due to their good comprehensive mechanical properties. The phase equilibria of the Mg-Zn-Zr system at 400°C covering the overall composition range were investigated by X-ray diffraction and electron probe microanalyses on thirteen ternary alloys. Three ternary compounds, s 1 , s 2 and s 3 , were detected to be thermodynamically stable at 400°C, and their homogeneity range was determined to be Mg (7-17) Zn (80-88) Zr (4-6) , Mg (15-22) Zn (66-68) Zr (9-16) and Mg 9 Zn 68 Zr 23 (in at.%), respectively. Eight three-phase regions and four two-phase regions were observed. The maximum solubility of Mg in Zn 22 Zr, Zn 39 Zr 5 and Zn 3 Zr phases was measured to be 0.52, 0.37 and 0.99 at.%, respectively, while the solubility of Zr in MgZn 2 and Mg 2 Zn 3 phases is negligible. The isothermal section of the Mg-Zn-Zr system at 400°C was then constructed based on the present experimental data.