A shear-lag model is developed to predict the stress distributions in and around an isolated fibre in a single-fibre polymer matrix composite (PMC) subjected to uniaxial tensile loading and unloading along the fibre direction. The matrix is assumed to be an elasto-plastic material that deforms according to J2 flow theory. The stress distributions are obtained numerically and compared with a different shear-lag model that employs total strain theory as a constitutive equation of the matrix material. An effect of the difference between the models on the derived stress state is discussed. Axisymmetric FEM is also conducted to validate the stress states obtained from the present analysis. It proves to be in good agreement with the FEM, which illustrates the importance of application of the incremental analysis.