CoSb3 with nanoscale was synthesized by Cross-coprecipitation. A precursor consisting of antimony oxide and cobalt hydrate was prepared by the reaction of CoCl2, SbCl3 and both precipitators at room temperature. The precursor was reduced in thermal treatment under hydrogen atmosphere whereby the CoSb3 was thus obtained. The parameters especially reducing temperature and atmosphere (content of H2) influence the constituent phases and particle size of product significantly. The single phase CoSb3 with the average grain size of 60~70 nm was obtained after reduced at 723 K for 2 h with pure H2. Nanoscale CoSb3 powder was used as starting materials, and bulk CoSb3 compound was prepared by spark plasma sintering (SPS). The effect of grains size on thermal conductivity was investigated.