An aqueous magnetic suspension was prepared by dispersing amphiphilic co-polymer (ACP) coated monodispersed magnetite nanoparticles (MNPs) synthesized through thermal decomposition of iron(III) acetylacetonate in a mixture of oleic acid and oleylamine. ACP composed of poly (maleic anhydride-alt-1-octadecene) and polyethylene glycol methyl ether was applied to disperse MNPs in water and buffer solution. The average diameter of MNPs increased from 6.6 to 12.5 nm with increasing reaction temperature from 200 to 250°C. Infrared spectra and elemental analysis revealed that the surface modified MNPs contain carboxyl groups originated from oleic acid and ACP. The conjugation of MNPs and bovine serum albumin (BSA) antibody was examined. As a result, the as-synthesized MNPs adsorbed BSA antibody effectively than the surface modified MNPs: the BSA antibody adsorption decreased with increasing cross-linker concentration.