Chitosan nanoparticles are bio-adsorbent that have been studied for adsorption of metals. To enhance their adsorption capacity, chitosan nanoparticles were modified with citric acid to supplement the functional groups with high affinity for Co(II) ions. In the present study, chitosan-citric (Cs–Cit) nanoparticles were prepared to form a new amide linkage, by grafting the amino groups of chitosan in the presence of carboxylic groups of citric acid that acts as a cross-linking agent. The as-prepared Cs–Cit nanoparticles samples were characterized using scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) techniques. The results showed that the cross-linking agent preserved during the chemical modifications. The adsorption capacity of the prepared nanoparticles for the removal of Co(II) ions in aqueous solution was studied in batch experiments and the maximum removal reached 79 mg g^-1 at optimum conditions.