A nonlinear constitutive model together with a mixed failure criterion for a single lamina is developed to simulate the behavior of composite laminates under uniaxial compression. In the model, fiber and matrix are assumed to behave elastic-plastic and the in-plane shear to behave nonlinear with a variable shear parameter. The damage onset for individual lamina is detected by a mixed failure criterion, which is composed of the Tsai-Wu criterion and the maximum stress criterion. After damage is taken place within the lamina, fiber and in-plane shear are assumed to exhibit brittle behavior and matrix to exhibit degrading behavior. This material model has been tested against experimental data and good agreement has been obtained.