The present invention is directed to methods for quantitatively measuring biomolecules by using magnetic nanoparticles. Through the use of the magnetic nanoparticles and the bioprobes coated to the magnetic nanoparticles, the biomolecules conjugated with the bioprobes result in the formation of particle clusters. By measuring the changes in magnetic properties resulting from the existence of the bio-targets, the amount of the bio-targets in a sample to be tested can be measured.