Ultrafine particles of pure amorphous cobalt were chemically synthesized by borohydride reduction of a cobalt salt in aqueous condition. This material were subjected to a controlled annealing in air atmosphere for about an hour at different temperature. 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 crystallization and magnetic properties of Co particles were examined. The crystallization temperature of amorphous Co particle was found to be 311C, which also confirmed from thermomagnetic studies by Thermogravimetric analysis. It was observed that crystallization process occurs only in the pure Co particles, which trend is not observed in Co/SiO2 samples. However magnetization value of pure Co and Co/SiO2 both are increasing with increase of annealing temperature. The magnetization reach a maximum 35 emu/g for Co/SiO2 sample, whereas 25 emu/g for pure Co sample. The variation of magnetization and coercivity as a function of temperature were also discussed.