A novel nanocomposite Fe(3)O(4)-Au nanorod (AuNR) was prepared and used as the substrate in the surface plasmon resonance (SPR) biosensor to detect goat IgM. Fe(3)O(4)-AuNR nanocomposites were synthesized by a method of seed-mediated growth, and further characterized by molecular absorption spectroscopy, transmission electronic microscopy (TEM), energy-dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The nanocomposites exhibit both magnetic property and exceptional optical property, which are beneficial to the antibody immobilization and the sensitivity of detection. The sensing membrane can be regenerated easily and the experimental procedure is simplified. Moreover, the Au nanorods show two plasmon resonance wavelengths defined as transverse mode and longitudinal mode, and the longitudinal plasmon wavelengths are more sensitive to the changes in the dielectric properties of the surroundings. Fe(3)O(4)-AuNR nanocomposites got a high sensitivity in detection of antibody-antigen immunoassay. In the optimal conditions, the biosensor based on Fe(3)O(4)-AuNR nanocomposites exhibits a satisfactory response to goat IgM in the concentration range of 0.15-40.00 μg mL(-1). However, the biosensor without Fe(3)O(4)-AuNR nanocomposites shows a response to goat IgM in the concentration range of 1.25-40.00 μg mL(-1). As a result, the sensitivity of the biosensor based on Fe(3)O(4)-AuNR nanocomposites is enhanced significantly.