Thermoelectric (TE) materials, which can convert waste heat into precious electricity, has been viewed as an effective solution for the worldwide energy issue. The simple binary compound, GeTe, has long been considered as a promising thermoelectric materials in the temperature range of 200℃-500℃. Phase diagram provides basic yet essential information that probes the thermodynamically phase stability behaviors of complicated materials systems. With an aid of phase diagram and microstructural evolution, the TE properties can be optimized. We aim to determine the ternary phase diagram of Ge-Te-In system. The efforts of this study include: (1) determining the liquidus projection by water-quenched alloys, (2) constructing the 350˚C isothermal section by thermally-equilibrated alloys, and (3) measuring the thermoelectric property of In-doped GeTe alloys. Metallographic observations upon the quenched or thermally-equilibrated alloys are conducted using SEM while the phase identifications and compositional analysis are carried out by XRD and EDS, respectively.