This study presents a novel polymer MEMS structure with Fe3O4-glycerol fill-in. Thus, the concept of magnetorheological effect is exploited to increase the stiffness of polymer MEMS structure by applying a magnetic field. The stiffness enhancement by magnetic field is achieved through the following mechanisms: (1) distribution of the Fe3O4 particles in glycerol would change from isotropic to anisotropic, and (2) the attraction between the aligned Fe3O4 particles. Note the stiffness of structure could also be reduced by varying the direction of magnetic field. In applications, parylene beams with Fe3O4-glycerol fill-in have been fabricated and tested. Preliminary results show the critical buckling loads of beams increase from 0.6gw to 1.5gw (1% fill-in), 0.78gw to 2.3gw (3.8% fill-in), and 0.78gw to 2.6gw (5 % fill-in) by 5mT magnetic field. The resonant frequency of beam is reduced for 1.2 kHz in as magnetic field applied in x-direction. However, the resonant frequency of beam decreased for 2-3.3% as the magnetic field applied at y-and z-directions.