Magnetic composite nanoparticle of Au/gamma-Fe2O3 was synthesized in an aqueous phase using gamma-ray. Connection between gold and gamma-Fe2O3 was confirmed by the magnetic separation technique. TEM observation shows that 5-nm gold particles were dispersed on 20-nm gamma-Fe2O3 particles. The nanoparticles adsorbed a water-soluble mercaptan, glutathione, and was manipulated by an external magnetic field.