Abstract: The behaviour of nonlinear stability of the stiffened plates under various inplane combined loadings is studied by variational principle and generalized Fourier series method.The equations are solved by use of Newton-gradient method.The pre-buckling compressing-bending coupling effect greatly affects the buckling and post-buckling behaviour of elastically supported composite stiffened plates so that the transverse deflection is produced when the compressing and shearing load starts to act on the plates.The pre-buckling coupling effect is not obvious only when the stiffened plates are clamped at four edges.Some relevant behaviour completely different from that of non-stiffened laminated plates is also obtained.The results predicted are in good agreement with the relevant calculated and experimental results in previous papers.