Abstract: Magnetic polystyrene microspheres were prepared by a modified suspension polymerization method and were characterized by scanning electron microscopy (SEM) and vibrating sample magnetometry (VSM). The number of the magnetic microspheres was detected with a giant magnetoresistance biosensor. The results show that the magnetic microspheres are spherical, their sizes range from 0. 5 to 50 μm, and their specific saturation magnetization is 4.56 A·m² ·kg^-l. The magnetic microspheres have a very good response to the sensor. With the amount of the magnetic microspheres increasing, the output signal of the biosensor intensifies. Under a given amount of the magnetic microspheres, the change in electrical resistivity of the sensor increases first and then decreases with the increase of the magnetic microsphere size.