Abstract: Based on the energy principle of buckling and the mixed hardening constitutive relation of orthotropic material, the formula of elasto-plastic critical stresses was derived and the value of critical stresses was calculated for simply supported orthotropic circular cylindrical shells under axial compression. The influences of geometry and material properties of a shell on its elasto-plastic critical stresses were discussed. The numerical examples show that the effect of geometry on the elasto-plastic critical stresses is not obvious while the effect of material properties on the critical stresses can not be ignored.