In this article an experimental study to determine the longitudinal (or in-plane) shearing stress-strain response of a unidirectional fiber-reinforced composite material is presented. The test method used is the four-point bending of a +/-45 degrees off-axis glass-fiber-reinforced laminate. Although a laminate is used for the investigation of the shearing stress-strain response, it is shown that unidirectional shear properties can be found from the laminate test data following a procedure analogous to that used in previously. also, the 45 degrees off-axis test of the unidirectional composite in bending was carried out to obtain the in-plane shear modulus and compare it with that obtained by the +/-45 degrees off-axis method. Finally both values were compared with the theoretical value of the in-plane shear modulus obtained from a theoretical formula where the concept of boundary interphase between fiber and matrix was introduced.