Using magnetic nanoparticles as labeling agents for detecting and quantifying the level of biological molecules are of interest in many biomedical applications recently. Owning to the high biocompatibility of their surfaces, the magnetic nanoparticles have the ability to conjugate specifically to the biological molecules of interest. Because of such specific binding, the concentration of the biological agent is determined by number of magnetic nanoparticles. A sensitive magneto-biosensor using giant magneto resistance sensing element together with a new sensing technique can determine accurately the number of the magnetic nanoparticles via a measurement of their induced magnetic field. It was demonstrated that the technique can help to detect and measure low biological concentrations in a timely fashion without the need of a special magnetic-shielding medium.