In this study, ferrofluids were prepared by colloidal dispersion of paramagnetic Fe₃O₄ nanoparticles coated with long-chain carboxyl terminated silicone oil in silicone oil. Effect of pH value of the aqueous solution on
coating was investigated. The IR and TG analysis indicated that acid conditions favored the coating of carboxyl terminated silicone oil. Stabilities of ferrofluids were characterized by density change and mass specific
saturated magnetization change during centrifugation and static settlement observation. The static settlement observation indicated that the prepared silicone-oil-based ferrofluids have better stability than unmodified and
oleic acid modified ferrofluids. Finally, the magnetic properties and stability of ferrofluids with different particle mass fraction were characterized.