In order to obtain better thermoelectric performance in the composition domain should be stabilized, the phase diagram of the Ag3-xSb1+xTe4 system by varying the Ag:Sb ratio. The phase diagram is investigated using the differential thermal analysis and the powder X-ray diffraction techniques. The Seebeck coefficient and the electrical resistivity of the grown bulk crystals of the system are also measured. The phase diagram of the Ag3-xSb1+xTe4 system indicates that a mixed phase of AgSbTe2 and Ag2Te, which is expected to show higher thermoelectric performance, exists in a wide temperature range between 600 and 830 K at a composition of Ag2.2Sb1.8Te4. The maximum of Seebeck coefficient for AgSbTe2 (x=1) is 0.73 mV/K at about 680 K. The thermoelectric performance is lowered by the compositional deviation from Ag:Sb:Te=1:1:2. (C) 2004 Kluwer Academic Publishers.