Our invention is a method for specifically detecting targeted molecules via magnetization contrast of magnetic particles induced by force. Magnetic particles with ligands to target specific molecules are loaded to biological samples to be diagnosed. By comparing the magnetic signals before and after exerting a mechanical or magnetic force on the sample, we are able to distinguish whether there exist the targeted molecules for the magnetic particles to bind to. Multiple molecular targets can be diagnosed and imaged by varying the amplitude of the force. The absolute quantity of each type of the targeted molecules is obtained with high sensitivity and selectivity.