Abstarct: In this research Polystyrene / NiO Nanocomposite was prepared in three stages. First, nickel peroxide was prepared from reaction of Ni(NO3)2.6H2O with sodium hypochlorite in the presence of surfactant cetyl trimethyl Ammonium Bromide (CTAB) in the alkaline environment and then its oxidation by ethanol, obtained Nickel oxide nanoparticles..Then, the surface of nanoparticles was modified in order to obtain better scattering and proper compatibility in organic environment by oleic Acid. These Modified Nanoparticles, were scattered in styrene monomer and through polymerization of mimiemulsion, nanocomposite Polystyrene/NiO nanocomposite was prepared. Nickel oxide nanoparticles and polymer nanocomposites were identified and confirmed by the techniques of Fourier Transform Infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM) and Energy-dispersive X-ray (EDA). Thermal stability of the nanocomposite was assessed by the techniquse of simultaneous thermal analysis (TG/DTA) and Differential Scanning Calorimetry (DSC) and the magnetism of nanocomposite was investigated by Vibrating Sample Magnetometer (VSM).studying of nanocomposite compared with pure polymer. also, Nickel oxide nanoparticles and polymer nanocomposite weak ferromagnetic properties