Various methods are proposed to obtain the in-plane shear stress-strain relation of unidirectional composite materials, but these are generally complicate and not always lead reliable results. This work proved that in-plane shear relation could be obtained from the only uniaxial tension test results of the ±45°symmetric angle-ply laminate, and presented an experimental technique taking into account the nonlinear behavior of the shear stress-strain relation for predicting the stress-strain response up to ultimate fracture for symmetric angle-ply laminate. The results of this analysis by using the laminate theory were found to agree fairly with that of the experiments.