Iron oxide supported on activated carbon (IO/C) was prepared by a process based on homogeneous precipitation. The iron oxide content and specific surface area of the IO/C were varied by varying the ratio between initial amounts of activated carbon and iron(II) chloride tetrahydrate. Citric acid was used as a dispersant of iron oxide particles on activated carbon. The size of the magnetite particles in the IO/C prepared in the presence of citric acid was decreased with an increase in the concentration of citric acid at a constant magnetite content. The saturation magnetization of the IO/C varied from 0.5 to 39.9 emu/g according to the composition, content, and size of the iron oxide particles.