Gold nanoparticles in size of 9.0nm was prepared by the trisodium citrate and used to label goat anti-human fibrinogen. In the pH 6.2 buffer solution and in the presence of polyethylene glycol (PEG), the immune reaction between gold-labeled goat anti-human fibrinogen and fibrinogen took place and the labeled gold nanoparticles were released from the goat anti-human fibrinogen, and the released gold particles aggregated which leaded the resonance scattering intensity at 560nm (I 560nm ) to enhance greatly. The I 560nm is proportional to the fibrinogen concentration in the range from 0.027 to 1.07μgmL −1 . The detection limit is 1.14ngmL −1 . This simple assay was applied to determination of fibrinogen in human plasma, with satisfactory results.