Abstract: Fiber bundle reinforced polymer composites(FBC) and unidirectional laminates demonstrate similar failure features in the standard Iosipescu shear tests. However, a considerable difference is observed between the measured shear strengths of the two kinds of specimens. In this paper, three types of FBC and unidirectional laminates were prepared using two kinds of carbon fibers and epoxy resins. The shear strength of FBC and unidirectional laminates were compared to explore the underlying mechanisms resulting in the different shear strength. It is found through the analysis of the interfacial stress field between fiber bundle and matrix using the interface element method that the interfacial stress state in FBC specimen is a coupling of tensile stress and shear stress, whereas the interface in unidirectional laminate specimen is in an ideal shear stress state. This difference makes the FBC fail at lower shear stress level. In this paper, a strength model was proposed to bridge the shear strength of FBC and unidirectional laminates using Yamada-Sun strength theory. The experimental verification shows that the shear strengths of the unidirectional laminate predicted by the proposed model reach a good agreement with the measured values, with the relative deviation of about 10%. This study provides an effective method to predict or evaluate the interlaminar shear strength of unidirectional laminates based on the tests of FBC with relatively simple sample preparation procedure.