The structures and thermoelectric properties of the double-filled (CaxCe1−x)Fe4Sb12 series (x=0, 0.25, 0.5, 0.75, and 1) have been studied using a combined experimental and computational methods. Compounds of (CaxCe1−x)Fe4Sb12 were obtained only for x=0, 0.5, and 1. Composition with x=0.25 was found to be a mixture of x=0 and 0.5 compounds, and composition with x=0.75 was found to be a mixture of x=1 and 0.5 compounds, respectively. Our conclusions on phase formation are supported by density functional theory (DFT) calculations. In Ca0.5Ce0.5Fe4Sb12, Ca substitution in the Ce site of CeFe4Sb12 leads to high hole concentrations, resulting in stronger semimetal transport as compared to CeFe4Sb12. Ca0.5Ce0.5Fe4Sb12 yields a slightly higher ZT value than that of CeFe4Sb12, which is attributed to its lower lattice thermal conductivity. Phonon mode calculations adopting a three-particle bending model suggest that thermal conductivity is reduced upon Ca substitution because of an additional vibration mode which involves both Ca and Ce atoms. Detailed structural information to correlate with thermoelectric properties in a series of double-filled (CaxCe1−x)Fe4Sb12 skutterudite samples were obtained using synchrotron X-ray diffraction and first principle calculations. Display omitted •Research focus on phase stability of CaCe double-filled p-type skutterudite.•Stable structure forms for an equiatomic occupancy of the voids by Ca and Ce.•Samples feature two-phase mixtures with variations of filling atoms ratio.•Phase stablility is investigated by DFT total energy calculations.•Sample with Equiatomic occupancy of Ca and Ce show higher ZT.