This paper deals with the effect of core deformations caused by a periodic external on the transmission characteristics of an optical fiber. For simplicity, the distribution of the periodic external force is assumed to be a sinusoidal, and the slab waveguide model is analyzed to clarify the loss mechanism of the round optical fiber under the periodic external force. In theoretical investigations, the Green's function for the displacements of the axis of a slab waveguide model is obtained. By using the Green's function of the displacements and the mode coupling theory, we clarify the relation between the radiation loss due to the core deformations and the external force. It is found that the radiation loss of the fundamental mode or the lowest order mode decreases as the spatial wave number of the periodic external force and the cladding radius increases. Experimental investigations on the excess loss of the single mode fiber have also been made, the results can be explained qualitatively by the theoretical results of the slab waveguide model.