Ferronickel slag is a by-product of nickel ore smelting that is categorized as hazardous and toxic waste to the environment that still contains valuable minerals such as nickel, iron, cobalt, and some rare earth elements. Further research is needed to become the solution of the environmental issues that this slag had caused and to find an alternative solution to the decrease in the world's minerals supply by re-using ferronickel slag. In this research, the ferronickel slag is first given alkaline fusion to create microcracks and to bind silica on the slag by mixing it with sodium carbonate and roast them at 1000°C and hold it for 60 min. Then followed by hot water leaching to separate silica from the slag at 100°C for 60 min. Next, the slag is leached by using 1,5 M sulfuric acid with the various temperature at room temperature, 50oC, 60°C, and 70°C with the various time that is 15, 30, 60, 90, and 120 minutes to get the most effective condition to dissolve the nickel and iron. The most effective condition to dissolve nickel and iron is expected at 70°C leaching temperature and 120 minutes of leaching. The result of this experiment is characterized by ICP (Inductively Coupled Plasma) to know how much nickel and iron had dissolved to the solution. The result of this research is 70°C leaching temperature and 120 minutes of leaching is the best condition to make the highest % of nickel and iron extraction.