•Valence state of doped Sb ions is variational as Sb-doping level increases.
•The spin-density-wave state is more stable in the Sb-doped samples.
•The ZT value of Ca3Co3.90Sb0.10O9 is two times larger than that of Ca3Co4O9.
•Sb-doping may be effective to improve thermoelectric performance of Ca3Co4O9.
The structure, magnetic, electrical and thermal transport properties of Ca3Co4-xSbxO9 (0⩽x⩽0.2) have been investigated systematically. Base on the analysis of X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), the valence state of the doped Sb ions is suggested to be +3 in the x = 0.05 sample, and its content increases monotonously along with the gradual introduction of Sb5+ ions as x further increases. The resistivity and the metal-insulator-transition temperature increase with increasing x, indicating the more stable spin-density-wave state in the Sb-doped samples. The thermopower decreases first as x = 0.05, and then increases monotonously with increasing x till to 0.15. The thermal conductivity decreases obviously due to Sb-doping. As a result, the Ca3Co3.90Sb0.10O9 sample has the largest thermoelectric figure-of-merit ZT value of 0.012 at room temperature, which is about two times larger than that of Ca3Co4O9. The results are suggested to originate from the variations of carrier concentration and electronic correlation via Sb-doping with the different valence states.