A technique to measure the shapes and corresponding radiation efficiencies of the radiation modes for vibrating structures is proposed. First, the radiation operator is imposed to analyze the radiation mode for arbitrary structures. Then the reciprocal method is imposed to obtain the experimental version of radiation operator. By placing a source with known volume velocity in the far-field and measuring pressure on surface of the structure can obtain the appropriate radiation operator. Final, with an example of a rectangular glass plate, experimental study was carried out to verify the proposed method. The shapes and corresponding radiation efficiencies for the first five radiation modes were measured. The experimental results show the feasibility of the proposed reciprocity method.