Molecular‐beam epitaxial growth of GexSi1−xon ⟨100⟩ porous Si substrate was studied using Rutherford backscattering spectrometry, transmission electron microscopy, and defect etching. This study was stimulated by the theoretically predicted possibility of stress field reduction in a lattice‐mismatched film grown on a patterned substrate. The experimental results show predominantly 60° dislocations with long misfit segments, and no reduction of dislocation density is observed by the use of porous Si substrates. The nature of interconnected growth area of the porous Si substrate is believed to be the reason for the ineffectiveness of the stress reduction.