Elasto-plastic buckling of orthotropic laminated plates, which include interfacial damage, is analyzed in detail. Firstly, a novel mixed hardening yield criterion, as an improvement of Hill's counterpart, is proposed for the orthotropic materials on the basis of the plastic theory. And differing from Hill's theory, the present yield criterion is related to the spherical tensor of stress. Then, the incremental elasto-plastic constitutive relations of the mixed hardening orthotropic materials are presented. Secondly, the incremental static equilibrium equations for laminated plates including interfacial damage are established based on Von-Karman type theory and the principle of minimum potential energy. Finally, the elasto-plastic buckling of laminated plates are solved by adopting the Galerkin method and iteration scheme. The numerical results show that buckling of the plate occurs easier due to the existence of interfacial damage, and the critical load trends to constant when the interfacial damage approaches a certain degree. Also, the effect of anisotropy on buckling is obvious and the analysis of elasto-plastic buckling is necessary.Laminated structures are extensively used in automotive and aerospace applications, and the stability problem of which is an important subject in the solid mechanics at all times. As these structures can still bear loads when exceeding the material yield limit, the traditional elastic design scheme generally used in engineering is conservative in safety consideration. So it is uneconomical and the more reasonable elasto-plastic analysis is necessary. On the other hand, due to the technology problems in the machine-shaping and some external factors, the layers are difficult to be bonded perfectly. The interfacial damage, which has evident influence on the mechanical property of structures, will always develop in the operating circumstance.Determination of elasto-plastic constitutive relation is one of the essential issues in the analysis of mechanical property for laminated structures, and researches have achieved great progress on this aspect [1][2][3][4]. Most of these models, however, are based on the Hill's yield criterion, which suppose that material yield is independent of the spherical tensor of stress. Actually, the structures will be distorted due to the different elastic constants in every principal direction under the acting of the spherical tensor of stress. According to the Mises distortion energy yield criterion, the materials will get into plastic stage if the distortion energy approaches a certain value. Moreover, the materials have apparent Bauchinger effect under the acting of the complex stresses. Hence, Hill's hypothesis is inconsistent with the practical situation.