Photoluminescence properties of as‐grown and post‐growth annealed GaAs directly grown on Si substrates by metalorganic vapor phase epitaxy are studied at various temperatures. At low temperature, three extrinsic lines and an intrinsic exciton line split by residual biaxial tensile stress are observed. The extrinsic lines give evidence of growth‐induced defects. One of these lines, involving the presence of Si acceptors, appears after post‐growth annealing (10 min at 800 °C). The biaxial stress deduced from the intrinsic lines varies with temperature; extrapolation to zero stress results in a temperature slightly below the growth temperature.