Te doping effect on the thermoelectric properties of solid solution was investigated. 2at% Te doping was found to be most effective to improve the dimensionless figure of merit, ZT, of pseudo-binary CoSb(3-4)mol%Zn4Sb3. Maximum ZT of 0.69 was obtained at 450K in CoSb(3-4)mol%Zn4Sb3 doped with 2at% Te. Heavy doping of Te above 3at% caused thermal conductivity to increase very high because of the formation of metallic CoSb2 phases and thus did not improve ZT.