The effect of free edges on the thermal stress distribution in square‐patterned GaAs/Si (100) is studied by three‐dimensional finite‐element elastic analysis. The results are discussed in comparison with previous analytical and numerical calculations. Finally the stress in the most important central portion of the square is calculated as a function of width to thickness ratio in the range 1 to 40. Results are presented in a form appropriate for interpretation of 4.2‐K photoluminescence (PL) measurements and device optimization.