This study presents the material design of Si1-xGex epitaxial films/Si for thin film thermoelectric generators (TFTEGs) by investigating their thermoelectric properties. The thermoelectric films composed of group-IV elements are advantageous due to their compatibility with the Si process. We fabricated Si1-xGex epitaxial films with various controlled x values and strains using various growth methods. Ge epitaxial films without strains exhibited the highest thermoelectric power factor (∼47 μW cm-1 K-2) among various strain-controlled Si1-xGex (x ≠1) epitaxial films, which is higher at room temperature than SiGe alloy-based bulks ever reported. On the other hand, strained Si1-xGex epitaxial films showed an ultralow thermal conductivity of ∼2 W m-1 K-1, which is close to the value for amorphous Si. In addition to strained SiGe films with the ultralow thermal conductivity, unstrained Ge films with a high thermoelectric power factor can also be used for future TFTEGs by applying a nanostructuring technique. A preliminary TFTEG of Ge epitaxial films was realized, which generated a maximum power of ∼0.10 μW cm-2 under a temperature difference of 20 K. This demonstrates that epitaxial films composed of group-IV semiconductors are promising materials for TFTEG applications.