Cobalt particle were synthesized by the polyol synthesis with sodium borohydride as the additional reductant and 1,10-phenantroline, etidronic, salicylic, succinic, tartaric and oleic acids as capping agents for varying particle size. The phase composition of the obtained samples was confirmed by X-ray diffraction (XRD), morphological studies performed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and specific surface area (SSA) estimation. The dependence of sizes and magnetic properties of the prepared nanoparticles on the synthesis condition (reaction time, capping agent nature and concentration) was investigated. The interaction between capping agents and particle surface was studied using FTIR spectroscopy. The catalytic activities of the obtained samples were evaluated for the aqueous reduction of 4-nitrophenol (4-NP) to 4-Aminophenol (4-AP) with sodium borohydride. The obtained results indicated the presence of reaction induction period, the minimum value of which is 1 min, while 90% conversion of 4-NP was observed after 12 min. We conclude that use of capping agent of different nature allows to regulate particle sizes and magnetic properties and improve their catalytic activity.