n-type Mg3Sb2-Mg3Bi2 alloy shows as a potential new thermoelectric material (TE) and has been widely researched recently. The pure phase n-type Mg-3.20(Sb0.3Bi0.7)(1.99)Te-0.01 were prepared by adjusting Mg content with the Bi impurity phase being effectively suppressed. Then, Co element was doped into the pure phase and the electrical conductivity of samples were improved. With a high power factor of 20.3 mu W cm(-1)K(-2) for Mg3.185Co0.015(Sb0.3Bi0.7)(1.99)Te-0.01 at 525 K. Additionally, it was found that the phonon scattering is enhanced due to the larger atomic mass of Co comparing to Mg and the lattice thermal conductivity is reduced. As a result, a high ZT value of similar to 1.03 at 525 K is achieved for the Mg3.185Co0.015(Sb0.3Bi0.7)(1.99)Te-0.01.