Abstract: Radioisotope thermoelectric generators (RTG) are widely utilized in deep space explorations. In order to enhance the output power of the thermoelectric modules and optimize their geometries, BiTe-based thermoelectric modules with different thermoelement thicknesses are prepared. An experiment setup is built to measure the output power and the match load of the thermoelectric modules along with the thickness of thermoelement under various temperature differences. According to the test results, as the thermoelement thickness decreases, the output power of the thermoelectric module increases linearly, meantime, the corresponding load resistance sees an opposite trend. The maximum output power of a thermoelectric module with a thermoelement thickness of 1.0 mm reaches almost 8.2W, and the area-specific output power is nearly 0.52W·cm^-2.