At present, the finite element method has become a standard tool directly and widely applied to ship structural design. One important application is the overall analysis of ship hull or a large portion of it to evaluate stresses acting on different members. Although accurate results may be obtained by the finite element method, the accuracy of these results depends on the way they are used. For example, when plate buckling strength under combined loads is evaluated approximate buckling interaction equations, the inaccuracy of these equations may diminished the value of the accurate stress analysis by the finite element method. In this paper, an accurate and efficient calculation method of the buckling strength of a rectangular flat plate is presented. The boundary condition is any combination of being simply supported and fixed, and the plate is subjected to combined two directional axial stresses (σ_x^aaf and σ_y^a), inplane shear (τ_xy) and two directional inplane bending (σ_x^b and σ_y^b). Biharmonic deflection function which satisfy the boundary conditions is assumed and made to satisfy the boundary conditions. The principal of virtual work is applied to formulate a general eigenvalue problem in a matrix form. Solution of this problem provides the buckling stress and the corresponding buckling deformation pattern. The accuracy of this method is estimated. The same method is used with any boundary and loading conditions, and is implemented in one compact and efficient computer program. The buckling values may be more accurate than those by the finite element method with less computer time.