The sensitivity of immunoassays strongly depends on the binding affinity of the antibodies, which is also limited by a variety of other factors that are specific to the assay configuration, the type of label and instrument system used for detection. Magnetic beads are used in order to increase sensitivity in diagnosis system using the piezoelectric microcantilever. Use of magnetic beads is seemingly viable to insure the high sensitivity as well as the stable immunoassay. The ultra-sensitive magneto-mechanic detection of protein is accomplished by monitoring resonant frequency change of a cantilever functionalized with magnetic beads. The resonant frequency is surely shifted by mass and magnetic field effect. As a result, we practiced sandwich assay using CRP secondary antibody-coated magnetic bead on cantilever. Resonant frequency of the cantilever is changed by mass effect. When CRP secondary antibody-coated magnetic beads are attached to a CRP antigen, the changes in normalized resonant frequency shift δf/f1 were found to definitely increase about 30 times.