Yb-doped n-type skutterudites have been widely studied in the past decade due to their good thermoelectric properties, yet there has been debate about the solubility limit of Yb in skutterudite phase. In this paper, a series of isothermal sections of the ternary phase diagram of Yb-Co-Sb system were studied, and different phase regions near CoSb3 were identified. The three-phase region on the Co-rich side of Yb-doped skutterudites (CoSb3, CoSb2 and YbSb2) results in a fixed skutterudite composition YbxCo4Sb12 with maximum Yb content at a certain temperature (for example x = 0.44 ± 0.01 at 700), whereas the three-phase region (CoSb3, YbSb2 and liquid) on the Sb-rich side leads to a fixed composition YbxCo4Sb12 with relatively lower Yb content at a given temperature (x = 0.26 ± 0.02 at 700). As temperature increases, the Yb content in both fixed compositions increases as well. This gives guideline for synthesizing samples with the optimum carrier concentration and thus optimized thermoelectric properties in a flexible composition range. The phase regions near CoSb3 and the temperature dependence of Yb solubility in skutterudite phase also explain the debate about Yb solubility.