To answer the fundamental questions of p-type skutterudites, systematical study on the influence of chemical composition on the electrical transport properties of RyFexCo4−xSb12 (R = Ce and Yb) has been carried out. By adjusting the filling fraction of fillers, the optimized electrical properties are obtained at the specific Fe content. It is found that the hole concentration increases with Fe content. Fe doping can also enhance the effective mass of holes significantly, which is beneficial for improving electrical performance. Because of the limit of electron supply, for trivalent Ce filling system CeyFexCo4−xSb12, the maximum figure of merit (ZT) value is achieved when Fe content is around x = 3, and for divalent Yb filling system YbyFexCo4−xSb12, the maximum ZT value is obtained even at lower Fe content. At high temperature above 700 K, the bipolar diffusion leads to great increase of total thermal conductivity and therefore deteriorates the thermoelectric properties.