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The Fe3O4/chitosan nanocomposite is one of the attractive functional materials. It can be applied as active material in a sensor system for an instant Surface Plasmon Resonance (SPR) based sensor. This research aimed at investigating the optical properties of Fe3O4/chitosan and its effect on the SPR angle. The synthesis of nanoparticles used the coprecipitation method. Encapsulation was carried out at a temperature of 50oC for about 60 minutes. The samples were characterized for their structural properties and phase composition by using X-ray diffraction (XRD) techniques. Their optical properties were characterized by using the Specular Reflectance UV-Vis Spectroscopy technique. The optical properties, such as absorbance (A), absorption coefficient (α), refractive index (n), extinction coefficient (k), optical gap energy (Eg), and Urbach energy (Eu) are investigated. The measurement of the SPR angle toward the ATR curve was carried out in a range of 30o–60o by using the Kretschmann configuration. The results show that encapsulation affects the optical properties of the nanoparticles. The SPR angle of prism/Au/Fe3O4/Air and prism/Au/Fe3O4-Chitosan/Air system is about 50.04 o and 50.93 o, consecutively. Chitosan affects an increase of the SPR signal toward the reflectivity where the SPR angle exists. These results imply a future prospective application of the Fe3O4/Chitosan as a signal amplifier material for the SPR sensor.