Ultrafine particles of pure amorphous cobalt were chemically synthesized by borohydride reduction of a cobalt salt in aqueous condition. This material when subjected to a controlled annealing in aqueous medium at 80 °C for about an hour yielded phase pure nanocrystalline particles of cobalt with a hexagonal close packed (hcp) structure as revealed by an X-ray diffraction (XRD) and trnasmission electron microscopy measurements (TEM) analysis. Silica coating was carried out using Stöber process. Pure cobalt and Co/SiO2 samples were annealed at various temperatures. The influence of thermal treatment on the crystal structure of Co particles was examined. The estimated crystalline size of as-prepared cobalt was 23 nm and increases with annealing temperature. The magnetization reach a maximum 96 emu/g. The dielectric constant of the sample is almost independent of frequency up to 1 MHz. Our method provides a promising route to fabricate magnetic materials in the highfrequency range.