We have fabricated several kinds of uni-leg thermoelectric (TE) modules using Sb-doped n-type Mg2Si. In order to evaluate the influence of the structure of the modules on their durability with respect to heat-cycling, modules of two different types were evaluated. One was a conventional-structured module, in which the upper and lower surfaces of the legs were each fixed to a ceramic substrate. The other was a ‘half skeleton’ module, in which the ‘cold-side’ substrate was removed and a thermal-conductive sheet was used instead of a ceramic plate for the cold-side insulator. From the result of this evaluation, it was confirmed that, although some variation in the output power was observed for the ‘half-skeleton’ module, the power variation was markedly less than for the conventional-structured module. Additionally, to improve the output power of the module, we replaced the Al2O3 substrate with Si3N4, which has a higher thermal conductivity than the Al2O3 substrate. The observed output power of a module (25 mm × 24 mm × 8.3 mm) fabricated using the Si3N4 substrate was 1,293 mW at ΔT = 500 K. The output value of the module using the Si3N4 plate was improved by 29 % compared with the output value of the module using the Al2O3 substrate. Moreover, based on the structures of these modules, a 36 mm × 41 mm × 8.3 mm module was fabricated. The expected value of the output power of the module was 1.9 W at ΔT = 500 K.