Stresses in passivated and unpassivated Al films with different thicknesses have been measured with x-ray diffraction. The stress strongly depends on the thickness of a film and is different from conventional elastic or plastic theory. A novel model based on plastic deformation and dislocation theories has been developed to explain it. In the model it is assumed that interaction between Al and the dielectric near their interface increases the yield strength of the Al him. Results calculated through the model are consistent with experimental results.