The enhancements in thermoelectric (TE) performances of p-type skutterudites are usually limited due to the relatively low Seebeck coefficients owing to the higher carrier concentration and more impurity phases induced by inherent structural instability of a Fe-based skutterudite. As shown in this study, alloying engineering of Ni doping at Fe sites in a p-type CeFe_3.8Co_0.2Sb₁₂ skutterudite can not only reduce the impurity phases with high thermal conductivity but also regulate the carrier concentration, and thus significantly increase the Seebeck coefficient. The thermal conductivity was largely suppressed due to the enhanced point defect phonon scattering and decreased hole concentration. As a result, a TE figure of merit ZT of the CeFe_3.5Ni_0.3Co_0.2Sb₁₂ sample reached 0.8, which is approximately 50% higher than that of a Ni-free sample. Appropriate Ni doping can maintain a high ZT at a high temperature by controlling the reduction in a band gap. Therefore, a high average ZT close to 0.8 at 650–800 K for CeFe_3.5Ni_0.3Co_0.2Sb₁₂ was obtained, which was comparable to or even higher than those of the reported Ce-filled Fe-based skutterudites due to the synergistic optimization of electrical and thermal performances. This study provides a strategy to synergistically optimize electrical–thermal performances of the p-type skutterudites by alloying engineering.