The Magnetic compound, Fe 3 O 4 , is one of the compounds that is easily hydrolyzed and oxidized. A coating method was presented in this study to prevent the process. The modifications of Fe 3 O 4 were carried out by supporting alginate as natural polymers on the surface of Fe 3 O 4. Preparation of magnetic alginate/Fe 3 O 4 (Alg/Fe 3 O 4 ) nanocomposite has been successfully synthesized. Alg/Fe 3 O 4 material was synthesized using the coprecipitation method with Fe 2+ ; Fe 3+ (with molar ratio 1:2) and alginate 1% as precursors. Synthesized Alg/Fe 3 O 4 nanoparticles were characterized by Fourier Transform Infrared Spectroscopy (FTIR), Delsa TM Nano C Particle Analyzer, Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), and Energy Dispersive Spectroscopy (EDS). The Alg/Fe 3 O 4 has an average diameter of 13.70 nm. Photodegradation test of synthetic methylene blue (MB) dye was found at pH 5.0 and irradiation time under UV 245 for 60 minutes. The degradation results showed that the percentage of MB dye removal reached more than 90%.