A convenient method to determine stress in thin films deposited on single crystalline silicon wafers is described. The film is treated as an elastic membrane attached to the edges of a thin circular disk. Due to the stress forces of the film, the substrate deforms elastically and assumes a parabolic curvature. This parabolic deflection is determined by measuring the substrate profile on the surface with a light section microscope. Knowing the curvature and the elastic constants of the substrate wafer, one can calculate the stress forces exerted by the film. The experimental accuracy of the method is about ±10%.