Functionalized nanoparticles and their assembly have been the subject of intense recent research activity. The stability and assembly of functionalized nanoparticles are still challenges for practical applications. Polymers are excellent stabilizing agents which can be used in nanoparticles synthesis. This dissertation describes polymer mediated nanoparticles which synthesized by both chemical and photo reduction techniques. The synthesis methods and assembly of polymer mediated nanoparticles were systematically studied. Two kinds of polymers were used in this research: PAMAM dendrimer and hyperbranched polyethylenimine. PAMAM dendrimer was used as the host for the synthesis of FePt nanoparticles with narrow size distribution by chemical reduction. Magnetic and structural properties of the as-made and annealed samples were studied. The as-made FePt nanoparticles have the chemically disordered fcc structure and can be transformed into the chemically ordered fct structure after annealing. But the annealing process caused both the size and size distribution to increase. To prevent size increase during annealing, dendrimer mediated alloys were encapsulated in silica microspheres. Their magnetic properties can be manipulated by synthesis conditions. Polyethylenimine is a kind of hyperbranched polymer, which was used to encapsulate nanoparticles. With polyethylenimine, Pt and Co₃O₄ nanoparticles were synthesized by photochemical processes. Their formation mechanisms were discussed. The chemical and electrochemical catalytic activity of polyethylenimine mediated Pt nanoparticles were studied. The results indicate that polyethylenimine is a good capping and reducing agent, and a carrier which improve samples stability and allows patterning. Ordered discrete magnetic nanoparticle arrays were also fabricated. Polyethylenimine was used as a stabilizer to synthesize magnetite nanoparticles. Polyethylenimine coating of magnetite nanoparticles can effectively prevent the aggregation of the nanoparticles. By combining the capillary filling technique with a magnetic field, polyethylenimine mediated magnetite nanoparticles form ordered assembly, in which the resolution of the pattern is doubled. The formation process of ordered structures was systematically investigated. This method is technologically feasible and scalable to make ordered magnetic nanoparticles over a large area. The result of this dissertation demonstrates that polymer stabilizers play an essential role in the synthesis of nanoparticles as well as their assembly.