The main portion of a ship structure is usually composed of stiffened plates. Between girders and floors, stiffeners are furnished to plates in one direction, usually the longitudinal direction. Under various loads applied to a ship, such as those due to waves, these stiffened plates are subjected to combined in-plane and lateral loads. In this report, buckling, ultimate and fully plastic strength interaction relations of plates and unidirectional stiffened plates subjected to in-plane biaxial and shearing forces, are derived and expressed in explicit forms based on the result of theoretical investigation of the nonlinear behaviour of plates and stiffened plates. The accuracy of the interaction relations is confirmed comparing with the result of analysis by other methods. With the aid of these interaction relations, buckling load and ultimate strength, or fully plastic strength of this type of stiffened plates subjected to in-plane loads may be predicted by hand calculation.