Skutterudite compounds InxCo4Sb12(x=0.1—0.4) have been synthesized by a melt-quench-anneal-spark plasma sintering method. x-ray diffraction (XRD) and filed emission scanning electron microscopy (FESEM) results show that doping of In results in a nano structured InSb phase distributed in the grain boundaries when the content of In exceeds its filling fraction limit in the skutterudites. Furthermore, the content of InSb increases with increasing In content. Our research indicates that the existence of nanostructured secondary phase InSb increases the power factor, decreases the lattice thermal conductivity, and therefore remarkably improves the thermoelectric properties of the compounds. The highest thermoelectric figure of merit ZT=1.21 is achieved at 800 K in the In0.35Co4Sb12 compound.