According to low temperature Ge buffer layer and selective area epitaxy technology, we selectively grow Ge film on patterned Si/SiO2 substrate using ultra-high vacuum chemical vapor deposition. By using X-ray diffraction (XRD), scanning electron microscope, atomic force microscopy and Raman scattering spectrum, we obtain its crystal quality and the laws of stress and other parameters varying with shape size. The results show that threading dislocation density decreases with shape size decreasing. Moreover, the tensile strain of Ge layer first increases and then turns stable with the increase of shape size, which can be attributed to the formation of (113) facet during Ge selective area growth. The formation of (113) facet reduces the strain energy of epitaxial material system, and the reduction of strain energy per unit volume decreases with increasing the shape size. The root-mean-square surface roughness of the Ge epilayer with a thickness of 380 nm is about 0.2 nm, the full-width-at-half maximum of the Ge peak of the XRD profile is about 678". It is indicated that the selective area epitaxial Ge layer is of good quality and will be a promising material for Si-based optoelectronic integration.