During a fiber pull-out test, it is desirable to analyze the stress profiles along the embedded fiber directly within the same time scale as the normal pull-out tests. In the present study, the axial tensile stress profiles of the fiber in a model composite are measured during the single-fiber pull-out tests by using stress birefringence of the fiber. It is concluded from the analysis of the measured stress profiles that an effective radius of matrix, i.e. a radius defining the region of the matrix where the major deformation takes place, is not constant but is an increasing function of the interfacial shear stress. By incorporating the variable values of the effective radius of matrix into the shear-lag model, the axial tensile and the interfacial shear stress profiles are calculated. To accurately estimate the interfacial shear strength, the stress distribution along the embedded fiber and the variability of the effective radius of matrix should be taken into account instead of calculating the interfacial shear strength simply from the pull-out stress and the embedded length.