A tetragonally distorted GaAs layer was grown on a Si(100) substrate by metalorganic chemical vapor deposition. From both the splitting and the shift of band-edge exciton lines in photoluminescence, the GaAs epilayer is found to be under biaxial tensile stress, which is quantitatively investigated using optical measurements, namely, Raman and temperature-dependent photoluminescence spectroscopy. The biaxial tensile stresses deduced from the intrinsic excitonic lines vary with temperature. The origin of the stress variation with temperature is discussed in detail. From the photoluminescence intensity ratios between the heavy- and light-hole excitonic transitions, the actual temperature where the conduction to heavy-hole band transition begins to predominate is found to be above 70 K.