Filled skutterudite compounds are known as excellent thermoelectric (TE) materials. It is known that the voids in the structure of the skutterudite compounds, such as CoSb3, can be filled or partially filled with a variety of different atoms, and, thus, obtained filled skutterudite compounds exhibit quite low thermal conductivity (kappa). In the present study, we tried to fill Ga into the voids of CoSb3. The polycrystalline samples of GaxCo4Sb12 (x = 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30) were prepared, and the TE properties were examined from room temperature to 750 K. All the samples were composed of two phases: GaxCo4Sb12 (x = similar to 0.02) as the matrix phase and Ga metal as the second phase. All the samples exhibited negative values of the Seebeck coefficient (S). The Hall carrier concentration slightly increased with increasing x, while the carrier mobility decreased. Although the maximum Ga filling ratio was really low, the kappa was reduced effectively by Ga adding. The maximum value of the dimensionless figure of merit ZT (= (ST)-T-2/rho/kappa, where T is the absolute temperature and rho is the electrical resistivity) was 0.18 at 500 K obtained for Ga0.25Co4Sb12. (C) 2011 American Institute of Physics. [doi:10.1063/1.3606417]