Filled skutterudites are known as excellent thermoelectric (TE) materials. The voids in the structure of skutterudites such as cobalt antimonide (CoSb<inf>3</inf>) can be filled or partially filled with a variety of different atoms. In the present study, we tried to fill both gallium (Ga) and indium (In) into the voids of CoSb<inf>3</inf>. The polycrystalline samples with nominal compositions of Ga<inf>0.2</inf>In<inf>x</inf>Co<inf>4</inf>Sb<inf>12</inf>(x = 0.15, 0.20, 0.25, and 0.30) were prepared and their TE properties were examined from room temperature to 773 K. Ga and In added to CoSb<inf>3</inf>partly filled the voids and most of the residual precipitated as nanoparticles at the grain boundaries. A tiny amount of Ga substituted for Sb at the Sb site of CoSb<inf>3</inf>, while it was more likely for In to fill the voids than Ga. All the samples exhibited negative values for the Seebeck coefficient. The lattice thermal conductivity was reduced effectively by the co-addition of Ga and In without degrading the power factor. The maximum dimensionless figure of merit (ZT) value of 0.95 at 725 K was obtained for the sample with the nominal composition Ga<inf>0.2</inf>In<inf>0.3</inf>Co<inf>4</inf>Sb<inf>12</inf>.