AbstractThe losses of optical fibers may increase during cabling or installation. Such a loss increase may be understood to be due to an exertion of unnecessary external forces on the optical fibers during the above processes because of improper fiber structure. This paper describes the result of the finite element analysis for the stresses applied in cross‐sectional direction upon the core fiber and the unit. It is expected that the degradation of transmission characteristics will be prevented if the stress less than an external force is applied to the fiber cores and unit. As for fiber cores, the 3‐layer cores having buffer layer are considered and then the optimum fiber diameter, buffer layer thickness, etc., are studied. In regards to the unit structure, the test is made for the unit in which a steel wire is located at the center, 6 or 10 core fibers are wound and a binder is applied. Using the above samples, the optimum binder wrapping thickness against the side‐wall pressure and the preferable relation between core fiber diameter and unit diameter are investigated. The core fibers and 6‐core unit are manufactured experimentally and the test is carried out to obtain the relation between the side‐wall pressure and transmission losses. The result shows excellent agreement with theoretical analysis. Thus, we use such a design method to minimize the losses of an opti