The paper presents the stability analysis of a sandwich plate of the shape of an isosceles trapezoid, subjected to unidirectional in-plane compression. The critical load value of the trapezoidal sandwich plate was obtained by a combination of the Galerkin orthogonalisation method and the proposed method of the coordinate system transformation. An influence of plate material and geometrical properties on the critical load level was analysed. The obtained results were verified in a numerical experiment conducted with the FEM ANSYS software package.