Ga-filled GaxCo4Sb12 skutterudite compounds with different Ga contents were synthesized by combining a melting quenching-diffusion-annealing procedure with spark plasma sintering (SPS). The effects of Ga content on thermoelectric properties were investigated. The results of Rietveld refinement indicated that the Ga is located in the 2a void site. The solubility limit of the Ga filling voids in CoSb3 was found to be close to 0.22. The electrical conductivity and the room temperature carrier concentration Np of the samples increase with the increasing Ga content for Ga filled GaxCo4Sb12 skutterudite compounds with x≤0.25, while the Seebeck coefficient decreases with the increasing Ga content. Room temperature Hall measurements show that each Ga atom donates approximately 0.9 electrons, which is significantly less than that of the Ga oxidation state (3+). Ga-filled skutterudites exhibit much lower thermal conductivity and lattice thermal conductivity in comparison with that of other partially filled skutterudites due to the smaller radius of Ga3+ ions compared with that of other filling atoms. The thermal conductivity and lattice thermal conductivity of Ga0.22Co4Sb12 compound are 3.05Wm-1·K-1 and 2.86Wm-1·K-1 respectively. The Ga0.22Co4Sb12 compound possesses the lowest lattice thermal conductivity at 600K. It is as low as 1.83Wm-1·K-1. The maximum Z value of 1.31×10-3K-1 is obtained at 560K for Ga0.22Co4Sb12.