Co9S8@C nanospheres with a 5 nm carbon shell and 50 nm Co9S8 core have been synthesized by a facile hydrothermal method followed by an annealing process. The electrochemical results indicate that the Co9S8@C nanospheres are an excellent anode material for sodium ion batteries. They deliver a specific capacity of 305 mA h g(-1) after 1000 cycles at current density of 5 A g(-1) and a high specific capacity of 405 mA h g(-1) at current density of 500 mA g(-1). The good cycling performance and rate capability are attributed to the synergistic effects of the carbon shell and the ultra-small size of Co9S8, which can enhance the electrical conductivity of the composite, as well as resisting the strain caused by repeated charge/discharge processes.