The behaviour of fiber-reinforced composites is examined through the formulation of a non-linear constitutive law obtained through a physical-based approach. The elasto-plastic macro constitutive equations for such a class of materials, composed by a matrix phase with elasto-plastic behaviour and several fiber-reinforcing phases, is obtained by considering an imperfect bounds between the matrix and the fibers, i.e. a certain amount of sliding is assumed to be present at the matrix–fiber interface. The fibers are assumed to have only axial stiffness with negligible relative matrix–fiber displacements orthogonal to the fiber's axis. The amount of sliding has been determined through energetic considerations by taking into account the evolution of the shear stress distribution along a single fiber during an increasing load history applied to the composite. The mechanical constitutive law has been implemented in a F.E. code and some numerical simulation have been performed in order to assess the reliability of the proposed model.