N-type Mg-3(Sb,Bi)(2) compounds have recently been demonstrated as promising low-cost efficient thermoelectric materials in low and intermediate temperature ranges; however, the thermal stability of this type of material still poses a great challenge for practical applications. In this work, we conduct a systematic investigation of the thermal stability of several high performing n-type Mg-3(Sb,Bi)(2)-based thermoelectric materials in both bulk and powdered forms using X-ray and neutron diffraction. It is found that the bulk sample exhibits a much slower degradation rate based on the evolution of the secondary Bi/Sb phase in comparison with the powdered sample, revealing a clear kinetic effect. Moreover, the surface of the bulk sample will gradually become Mg-poor or Bi-rich even at room temperature when exposed to air for a long time, highlighting the importance of surface encapsulation for applications. An underlying mechanism based on the Mg loss/migration is proposed to account for the property degradation. Importantly, to address the property degradation, we discuss possible solutions and propose Mg-vapor annealing as an effective approach to enhance thermal stability by suppressing the Mg loss/migration through saturating grains and grain boundaries with elemental Mg. We expect a combination of the Mg-vapor annealing and surface coating to further improve the long-term thermal stability. This work sheds light on the strategies for enhancing the long-term stability of n-type Mg3Sb2-based thermoelectrics for practical applications.