Patterned epitaxial GaAs films have been formed on Si substrates by either growth over patterned substrate (selective -area epitaxy) or chemical etching of patterns after growth. The optical properties of these samples are studied by 77K photoluminescence (PL) and the defect structures are investigated by transmission electron microscope (TEM). The patterned substrate consisted of bare Si stripes with width ranging from 10 µm to 100 µm surrounded by Si3N4 films on both sides and a reference area of bare Si. For 1.5 µmiann d 3 µm thick films, PL intensities from the films inside the 10 µm stripe shows 140% and 75% increase over unpatterned areas while the residual tensile stress in the patterned films is very similar to that of the unpatterned area. The increase in the photoluminescence intensity is ascribed to the reduction of crystalline defects inside the the window area. In the chemically etched sample, the pattern consisted of 4 µm by 4 µm squares and 1 mm long stripes with widths ranging from 100 µm to 4 plm. From the shift of PL peaks, a monotonic decrease in the tensile stress versus stripe width is observed. In particular, when the width of the stripe is less than 7 µm. tensile stress becomes essentially uniaxial in agreement with the results obtained by Yacobi et al [16] on a GaAs on InP sample. The polarization of the luminescence spectra parallel and perpendicular to the uniaxial stress of a 4 µm wide stripe agrees well with theoretical prediction. It is also observed that tensile stress is almost completely relieved in the 4 µm by 4 muentc hed squares.