Well-dispersed nickel nanoparticles in polystyrene were obtained by a sonochemical method. The proper ties of the as-prepared nanocomposite materials were characterized by XRD, TEM, EDAX, TGA, DSC, and a vibrating sample magnetometer (VSM). The nickel particles were 5 nm in diameter and were very well dispersed in the polystyrene. The magnetization measurements established that the as-prepared nanocomposite materials are superparamagnetic due to their small size. The saturation magnetization (30.1 emu/g) and coercivity (5 Oe) of the materials were significantly smaller than were those of the bulk nickel, reflecting the nanoparticle nature.