The magnetic nanoparticles modified with carboxyl functional group were synthesized and characterized. These nanoparticles covalently bound with hepatitis B surface antibody (HBsAb), were used to detect hepatitis B surface antigen (HBsAg) in immunovoltammetry. The detection limit was found to be 0.06 ng/mL, which is much higher than that of enzyme-linked immunosorbent assay (ELISA) used in clinical analysis.