This work is focused on the application of the calculation of phase diagrams method for alloy and heat treatment design. We analyzed the influence of Zn content on the precipitation of Mg(2)Sn in Mg-Sn-Zn alloys. A comparison with previous studies in the Mg-Sn-Zn system was made according to the published results and computational thermochemistry simulations. The phase evolution in the Mg-Sn-Zn system was evaluated for the different compositions, and the simulations were used for precise alloy and heat treatment design. The composition of the ternary alloy was set as Mg-8wt%Sn-1.25wt%Zn. The Sn and Zn content was designed and confirmed to be within the alpha-Mg solubility limit at the solution treatment temperature. The addition of Zn and the heat treatment applied resulted in the enhancement and refinement of the Mg(2)Sn precipitation. Three Vickers micro-hardness maxima were detected: precipitation of metastable Mg-Zn phases, heterogeneous precipitation of Mg(2)Sn on the Mg-Zn precipitates, and Mg(2)Sn precipitation in the alpha-Mg matrix. The CT simulations were found to be a valuable alloy design tool.