Structure and lattice deformation of porous silicon layers and silicon wafer bending as a result of high, temperature treatment in H₂ atmosphere at 900 to 1200 °C are investigated by SEM and X‐ray analysis. A significant expansion of the Si lattice of (2 to 4) × 10⁻⁴ is observed to appear in as‐grown porous silicon. It results in arising of elastic stress and bending of the wafer. High tempreature treatment leads to the evolution of a porous silicon structure, compression of porous silicon lattice, and reverse of wafer deformation. Porous silicon after annealing is found to be crystalline with spheroidal voids of 50 to 500 nm. A model based on the capillary mechanism and sintering theory is developed to explain the experimental results.