The mechanism of the internal stresses inside thin films is studied and a method is established to compute the stresses using the revised Thomas-Fermi-Dirac (TFD) model. It is the continuous boundary condition of electron densities across interfaces of composite films that is responsible for the appearance of large stresses. The results of the calculation are in agreement with the observational. The mechanism of the internal stresses inside thin films is studied and a method is established to compute the stresses using the revised Thomas-Fermi-Dirac (TFD) model. It is the continuous boundary condition of electron densities across interfaces of composite films that is responsible for the appearance of large stresses. The results of the calculation are in agreement with the observational.