The thermal stability of the interfaces between metal electrodes and thermoelectric (TE) materials is a significant factor for determining the reliability of intermediate temperature (300–800 °C) TE devices (TEDs). The interfacial cracks and elemental inter-diffusion cause power degradation of TEDs. To enhance TE device reliability, metallization layers should be inserted between electrodes and TE material. Metallization layers act as a diffusion barrier by suppressing elemental inter-diffusion and a stress damper by matching coefficient of thermal expansion (CTE). In this paper, we introduce a Co0.6Mo0.4 metallization layer for n-type skutterudite (SKD-N). The Co0.6Mo0.4 metallization layer inhibited the elemental inter-diffusion to less than ∼10 μm, and the electrical specific contact resistance (SCR) remained within the range of ∼10−6 Ω cm2 after thermal aging, which shows that the Co0.6Mo0.4 metallization layer has excellent interface reliability compared to a Ti metallization layer. These results demonstrate that the Co0.6Mo0.4 composite is a promising candidate as a metallization layer for the fabrication of highly reliable TEDs.