Thermoelectric properties of tin-filled skutterudites, SnxCo4Sb12, 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 SnxCo4Sb12 is compared to that of other filled skutterudites. It is concluded that tin atom is a better "rattler" in the CoSb3 host lattice.