The skutterudite structure and thermoelectric properties of CefFe8-xCoxSb24 were investigated by means of X-ray diffraction (XRD) experiments, energy-dispersive X-ray (EDX) measurements under a transmission electron microscope, and measurements of the thermoelectromotive force. The filling number of Cc into voids in the skutterudite (f) is variable at each substitution number of Fe by Co (x) and decreases with increasing x, The four-cornered ring of Sb tends to become square more rapidly with increasing f than with increasing x. The lattice parameter increases with increasing f; whereas it decreases with increasing x, The sign of the thermoelectromotive force depends on x for f < 1, although it is always positive for f > 1. The type of semiconductor changes at a critical valence electron density of 194/nm(3).