Continuous converting of copper matte based on calcium ferrite slag of the CaO-FeOx-Cu2O system is an attractive technology for copper smelting. This slag containing approximately 20 mass% CaO can prevent the excess magnetite precipitation, but sometimes tends to separate CaSO4 under a high oxygen potential required to produce copper metal. To clarify the mechanism of gypsum formation, the precipitation of CaSO4 in the CaO-FeOx-Cu2O slag system equilibrating with blister copper has been determined at 1250°C under the SO2 partial pressures of 0.2, 0.6 and 1 atm. It was found that the formation of CaSO4 in the CaO-FeOx-Cu2O slags is enhanced with increasing SO2 partial pressure or CaO content and also increasing oxidation degree of the blister copper. The behavior of CaSO4 formation in the slag is also discussed thermodynamically based on the free energy calculations combining with the activity data derived from Takeda's data on the CaO-FeOx-Cu2O ferrite slag diagram. Both of experimental and thermodynamical results show good agreements in general.