We simulated the performance of a planar-type thermoelectric generator with different thicknesses of silicon thin-film device layer. We introduced a variety of thermal conductivity depending on the thickness of silicon thin-film for both single-crystalline and polycrystalline in the simulation and found optimal dimensions of device design. We clarified that different dependence on thick-ness for thermal conductivity between single- and poly-crystalline silicon leads to a different optimal thickness of silicon device layer.