It was studied to extract silver from the last zinc-plant residue contained 340 to 650 g/ton of silver and the following results were obtained. 1) When the residue was calcined about 400℃, basic sulfates of iron (III), main component of the residue, begun to decompose to Fe₂(SO₄)₃and Fe₂O₃, and at about 700℃ Fe₂(SO₄)₃decompose to Fe₂O₃ 2) On calcining above 500℃, insoluble silvers (Ag₂S and Ag) in the residue were converted to sulfate by the reaction with Fe₂(SO₄)₃, produced by thermal decomposition of basic sulfate. The reactions and their standard free energy changes were as follows. Ag₂S(β)＋4/3Fe₂(SO₄)₃(s)＝Ag₂SO₄(s)＋4/3Fe₂O₃+4SO₃ ΔG°＝111,300－182.8T cal (452－933°K) 2Ag(s)＋2/3Fe₂(SO₄)₃(s)＝Ag₂SO₄(s)＋2/3Fe₂O₃(s)+SO₂ ΔG°＝40170－58.3T cal (298-933°K) 3) When the residue was calcinded in the temperature range between 550 and 650℃ for 20min., more than 85％ of silver was extracted with water at room temperature. Calcined at 600℃ for 10min., 90％ was extracted. On calcining above 700℃ silver extraction was decreased linearly with increasing calcination time.