Ge 2 Sb 2 Te 5 is a famous phase-change memory material for rewriteable optical storage, which is widely applied in the information storage field. The stable trigonal phase of Ge 2 Sb 2 Te 5 shows potential as a thermoelectric material as well, due to its tunable electrical transport properties and low lattice thermal conductivity. In this work, the carrier concentration and effective mass of Ge 2 Sb 2 Te 5 are modulated by substituting Te with Se. Meanwhile, the thermal conductivity reduces from 2.48 W m −1 K −1 for Ge 2 Sb 2 Te 5 to 1.37 W m −1 K −1 for Ge 2 Sb 2 Te 3.5 Se 1.5 at 703 K. Therefore, the thermoelectric figure of merit zT increases from 0.24 for Ge 2 Sb 2 Te 5 to 0.41 for Ge 2 Sb 2 Te 3.5 Se 1.5 at 703 K. This study reveals that Se alloying is an effective way to enhance the thermoelectric properties of Ge 2 Sb 2 Te 5 .