Stress in an optical fiber has not been measured because of its small diameter. This paper explains the protective quality of a jacketed fiber from lateral pressure by a new experimental method, which enables the stress in the fiber to be measured using the photoelastic effect. Results show that the effect of the nylon jacket with a silicone rubber layer depends on the shell effect of the nylon jacket within its elastic deformation region and is so large that little lateral pressure is actually applied on the fiber. However, fiber stress and excess loss increase suddenly when the nylon jacket shell yields.