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<p>This paper describes a series of compression tests performed on longitudinally stiffened plates fabricated from mild steel plates of thickness 4.0 mm with nominal yield stress of 240MPa. The stiffened plates with a range of rigidities of longitudinal stiffeners were tested to failure. The ultimate strength and performance of the longitudinally stiffened plate in compression undergoing distortional buckling or interaction between local and distortional buckling were investigated experimentally and theoretically. The compression tests indicated that the critical buckling mode was dependent mainly on the rigidity of the longitudinal stiffeners and the width-thickness ratio of sub-panels. A noticeable interaction between local and distortional buckling was also observed for some stiffened plates. A significant post-buckling strength reserve was shown for those sections with distortional buckling or with interaction between local and distortional buckling. A limiting strength curve for distortional buckling for stiffened plates was studied. Simple design strength formulae in the Direct Strength Method have been proposed to account for the interaction between local and distortional buckling of stiffened plates. The strength curves proposed were compared with the test results. The adequacy of the strength curve was confirmed. A set of conclusions was drawn from the experimental studies as to the buckling behaviour of stiffened plates.</p>