The normal stress distribution in the mid-surface of the stiffened plate at vibration is investigated theoretically, and using these results, the effective breadth for the typical vibrating modes are made clear. The normal stress distribution of the simply supported rectangular plate stiffened by a inverted angle on its center line is investigated theoretically and experimentally. The experimental results agree fairly well with theoretical ones. In general, the formulas of the effective breadth of vibrating stiffened plate are same as the ones of the stiffened plate subjected to the statically distributed sine load, but in the case of mode that all the stiffeners do not laterally deflect, but rotate torsionally about the intersection at vibration, the effective breadth takes the indeterminate form.