There are fire safety concerns in curtain walled buildings with a large glass window to wall area ratio. Thermal stresses of glass panels exposed to fire are a key problem and will be studied in this paper. Based on some experimental data available in the literature, the temperature profile of window glass panels will be deduced. Thermal stresses along the glass panel can then be calculated by taking the temperature profiles as boundary conditions. Results are useful to predict the positions on the glass panels where cracking is likely to occur upon exposure to fire.