In order to explore the possibility to extract copper from electronic scraps, a high temperature melting process was adopted to investigate the influence of slag viscosity on the gravity separation of copper from the liquid slag. For the purpose of optimizing the separation condition of the copper from slag by gravity, quaternary Al2O3-CaO-SiO2-10wt%MgO slag systems were chosen as basic slag compositions. Especially, two slag compositions, 15wt%Al2O3-29wt%CaO-46wt% SiO2-10wt%MgO and 20wt%Al2O3-35wt%CaO-35wt%SiO2-10wt%MgO, were chosen for the comparisons since they have been known to have low slag viscosity in the temperature range of 1300 and 1500 degreesC. From the experiments viscosities of 15wt%Al2O3-29wt%CaO-46wt%SiO2-10wt%MgO slag were measured to be 7.2, 16.5 and 52 poise at temperatures of 1500, 1400, and 1300 degreesC, whereas those of 20wt%Al2O3-35wt%CaO-35wt%SiO2-10wtMgO were 3.06 and 6.92 poise at temperatures of 1500 and 1400 degreesC, respectively. It was found that the slag viscosity made an influence on the gravity separation of copper from the slag and that less than 5 poise were enough for nearly 99% extraction of copper from the slag in 30 minutes by gravity separation.