Hydroxyapatite nanoparticles were synthesized by using the precipitation method with simulated body fluid solution and applied for adsorption of Co2+ and Sr2+ in aqueous solution. The single- and bi-solute adsorption experiments were performed to evaluate the maximum adsorption capacity of hydroxyapatite nanoparticles for Co2+ and Sr2+, the effect of temperature and pH, and the influence of simultaneous presence of other competing metal ion in a binary system. The synthesized hydroxyapatite nanoparticles had high adsorption capacity for Co2+ and Sr2+ due to a high specific surface area. The maximum adsorption capacity and binding energy of Co2+ were higher than those of Sr2+ in single-solute adsorption. The extended Langmuir model was fitted well for bi-solute competitive adsorption of Co2+ and Sr2+ onto the hydroxyapatite nanoparticles. Thermodynamic analysis showed that adsorption occurred spontaneously for both metals and was endothermic for Co2+, but exothermic for Sr2+.