(Ce_1-zNd_z)_yFe_4-xCo_xSb₁₂ skutterudites were prepared by encapsulated melting, annealing, and hot pressing. The formation of a marcasite phase (FeSb₂) was significantly suppressed by partially filling the voids with Ce/Nd and making a Co substitution at the Fe site. Both the Hall and the Seebeck coefficient values were positive, indicating that all specimens had p-type characteristics. The electrical conductivity decreased with increasing temperatures, as in a degenerate semiconductor. The Seebeck coefficient increased with the charge compensation of Co and filler content of Nd; however, the electrical conductivity decreased. The partially double-filled specimens had a higher electrical conductivity and lower Seebeck coefficient than the completely double-filled specimens, due to the increase in carrier concentration. The lattice thermal conductivity was expected to decrease via partial double filling with Ce and Nd; however, it did not change significantly because of the comparable atomic masses and ionic radii of Ce and Nd. Therefore, the partially double-filled specimens had lattice thermal conductivities comparable to those of the completely double-filled ones. Co substitution had an influence on the ZT values due to the change in the intrinsic conduction onset temperature, which decreased with increasing Co content. Thus, the (Ce_1-zNd_z)_yFe_4-xCo_xSb₁₂ skutterudites are a promising material for thermoelectric power generation in the mid-to-low temperature regions. (Received March 14, 2018; Accepted April 23, 2018)