Abstarct: Recently, magnetic nanoparticles are widely used as catalyst bed. For this reason nanoparticles have features such as high stability and high strength, easy recovery, biocompatibility and low toxicity. In this study, the Fe3O4 magnetic nanoparticles were synthesized by solvothermal method. Then the polyaniline was coated onto Iron oxide magnetic nanoparticles (Fe3O4/PANI) with p-toluene sulfonic acid (Fe3O4/PANI/p-TSA) or hydrochloric acid (Fe3O4/PANI/HCl) dopping. The molybdenum complex was then supported onto the Fe3O4/PANI/p-TSA nanocomposite and a heterogeneous catalyst Fe3O4/PANI/MoO2(acac)2 was prepared. The surface of the Fe3O4/PANI/HCl nanocomposite was modified with sodium-4-amino-5-hydroxy-7-sulfonaphthalene-2-sulfonate (AHS) and it used to stabilize silver nanoparticles (Fe3O4/PANI/AHS/Ag). The structure and properties of Fe3O4/PANI/MoO2(acac)2 and Fe3O4/PANI/AHS/Ag nanocomposites were assessed by FT-IR, XRD, VSM, ICP, FE-SEM and EDX. The Fe3O4/PANI/MoO2(acac)2 catalyst was used in the epoxidation of cyclooctene and a wide range of aromatic and aliphatic alkenes, using t-BuOOH as oxidant. This catalyst was easily separated from the reaction mixture by using an external magnetic field (magnet). The catalyst recovery was performed 4 times and the reaction efficiency decreased by only 3% during the recovery steps. In addition, the biological activity of Fe3O4/PANI/AHS/Ag nanocomposite was investigated against two species of plant pathogenic fungi called Fusarium graminearum and sclerotinia sclerotiorum and two species of plant pathogenic bacteria called Pseudomonas syringae and Xanthomonas sp. . The results showed that the growth of fungi and bacteria after 24 hours of contact with the nanocomposite is controlled and thus its biological activity was confirmed. The catalytic activity of Fe3O4/PANI/AHS/Ag was also investigated in reduction of nitroaniline. baxsed on the results, the prepared nanocomposite had the highest catalytic activity to reduction of 4-nitroaniline and 2-nitroaniline in 120 and 300 seconds, respectively.