PbTe and Sb2Te3, two of the most common thermoelectric materials, have stable mid- and high-temperature thermoelectric properties. Both can be tuned as either N-type or P-type semiconductors. The figure of merit (ZT) values of the PbTe and Sb2Te3 thermoelectric materials currently available on the market are around 0.8 and close to 1, respectively. In this study, Pb18Sb5Te20 was used as a substrate and Ag was then added to form an Ag0.6Pb18Sb5Te20 alloy. A comparison of thermoelectric properties between test pieces annealed at temperatures of 200, 400, and 600 ℃ and unannealed ones was conducted. The results revealed that the Ag0.6Pb18Sb5Te20 alloy annealed at 600 ℃ had the best thermoelectric properties. The experimental methods used in this study included the preparation of bulk material using vacuum arc melting, X-ray diffration (XRD) analysis, a comparison with the Joint Committee on Powder Diffraction Standard (JCPDS) database to confirm the face-centered rhombohedral crystal structure, and a study of thermoelectric properties such as the Seebeck coefficient, thermal conductivity, and resistivity. The objective is the improvement of the material thermoelectric figure of merit ZT.