The use of optical fiber sensors to study the drag force in molding flow is described. Research has proved that this novel method can be used to study the drag force of the molding flow in IC packaging over a perfectly elastic circular cylinder. The result of this study is found to be useful for characterizing the behavior of the wire sweep in encapsulation of semiconductor chips. An inline fiber etalon (ILFE) as a sensor is designed, constructed, and implemented in the middle of the optical fiber for accurate strain measurement. This is done by first laying an optical fiber in the midplane of a simple rectangular mold cavity, and then measuring the strain at the midspan of the fiber when it is subjected to the flow of a homogeneous fluid. For a given flow field, several dragforce models have been tried to calculate the drag force on the optical fiber, and the resulting strain of the optical fiber has been calculated by finite-element modeling and compared with the experimental results. From the comparison, the Takaisi model to calculate the drag force exerted on the optical fiber by the flow of homogeneous fluid is modified according to the experimental data. This modified model can be applied to study the wire sweep in the encapsulation of semiconductor chips. It is concluded that this measurement system can be used to verify the dragforce model for a molding flow on a perfectly elastic circular cylinder. Even under the more complex conditions in actual molding flow, the proposed sensing system can be used to measure the axial strain accurately. In addition, the transient deformation of the optical fiber characterized by the ILFE sensor provides important information for developing a drag-force model that can be used in IC packaging.