In this work, high-temperature annealing above 1073 K has been carried out to study the possible shape change of nanoporous Si thin films. Under 1273 K, it is found that the pore size can still be largely maintained when the nanoporous Si thin film is on a SiO₂/Si substrate. However, the pore size can significantly shrink when the film is suspended. The contrast suggests that the SiO₂/Si substrate can play an important role in maintaining the nanoporous patterns at a high temperature.  This finding can be important for the high-temperature applications of these porous thin films, such as thermoelectric power generation.