The purpose of this study is to enhance the sensitivity of Surface Plasmon Resonance (SPR) sensor using core-shell Fe3O4@Ag nanoparticles (NPs). Fe3O4@Ag NPs were synthesized by co-precipitation method with various concentration of Ag as a shell. The crystal structure of Fe3O4 corresponds to the cubic inverse spinel structure. The particle size of Fe3O4@Ag NPs with Ag concentration of 40mM is 13.8 nm. The saturation magnetization (Ms), and coercivity field (Hc) of Fe3O4@Ag NPs with Ag concentration of 20mM is 52.9 emu/g and 157.2 Oe, respectively, and then decreased with the increase of Ag concentration. An intensity of absorption peak increased with the increase of Ag concentration. A spherical nanoparticle consisting of a spherical Fe3O4 core covered by an Ag shell, was used as an active material to increase the signal detection of SPR, with a wavelength of 632.8 nm in the Kretschmann configuration. The system consists of a three-layer materials, i.e., prism/Au/Fe3O4@Ag NPs. The results show that the SPR angle shifted to the larger angle of incident light by using Fe3O4@Ag NPs and the addition of a core-shell in the conventional SPR-based biosensor leads to the enhancement of the SPR biosensor sensitivity.