Thermoelectric properties of tin-filled skutterudites, Sn_xCo₄Sb₁₂, are evaluated. The tin-filled compounds were synthesized under high pressure and temperature condition from powder mixture of CoSb₃ and elemental tin. The tin-filled compounds exhibit n-type semiconducting behavior indicating that the incorporation of tin atoms causes the formation of a donor level. A remarkable reduction in the thermal conductivity was achieved by tin insertion. Thermal conductivity of the tin-filled sample is two orders of magnitude lower than that of the unfilled CoSb₃. Effect of void filling on the thermoelectric properties of Sn_xCo₄Sb₁₂ is compared to that of other filled skutterudites. It is concluded that tin atom is a better "rattler" in the CoSb₃ host lattice.