Abstract: The fiber/matrix interfacial shear stress in the single-fiber reinforced composite was studied by photo-elastic experiment and the finite element method (FEM). Three different load levels were considered. It is found that the interfacial shear stress has a maximum value nearby the embedded top of the fiber/matrix interface. Along the fiber embedded length direction, the interfacial shear stress decreases quickly until it reaches a minimum value in the middle of the fiber. At the end of the fiber, there is a little increasing of the interfacial shear stress. It can be proved that the interfacial stress transformation from fiber to matrix or from matrix to fiber is mainly focused on the region of the embedded top of the fiber/matrix interface, where the interfacial shear stress reaches the critical stress first. In this region, the crack will initiate. Comparing the result of photoelastic experiment with the calculation of FEM simulation, both have the similar interfacial shear stress distribution.