Experiments about intrinsic stress in a porous silicon layer are described. After formation of a porous silicon layer compressive stress of 5 to 8 x 10(8) dyn/cm(2) is generated. By heat-treatment, the stress is changed from compressive stress to tensile stress at about 300 degrees C. This change is due to hydrogen bond dissociation, as shown by the decrease of the Si-H-2 stretching band of the porous silicon layer. The generation mechanism and the change of explained by the effect of hydrogen bonds and the surface tension of micropores.