Fiber optical strain sensors possess several advantages such as light weight, small dimension, high temperature endurance, dielectric nature, and immunity to electromagnetic interference, that meet the basic requirements to be a smart structure sensing element. As a sensor, it is expected that the strains between the optical fiber and host structure are the same. However, due to the existence of the adhesive layer and protecting coating, part of the energy would convert into the shear deformation. Thus, the strain of the optical fiber is different from the host structure. In this paper, experimental tests are performed to reveal the differential strains between the fiber-optic sensor and test specimen. Mach-Zehnder interferometric type fiber-optic sensor is adopted to measure the strain. The direct peak counting method is used to calculate the induced strain in the fiber-optic sensor. An electric strain gauge is attached to the test specimen to measure the strain in the specimen. Experimental results show that the strain measured at the optical fiber is lower than the true strain in the test specimen. The percentage of the strain in the test specimen actually transferred to the optic fiber is dependent on the bonded length of the fiber. Parametric study shows that the longer of bonded length the more strain is transferred to the optic fiber.