Abstract: A new crack diagnosis method for a two-story box structure based on the empirical mode decomposition is proposed in this paper. According to the FEM simulation analysis, it is shown that the modal of the structure can be barely influenced by the crack. The response signals of the swept sine vibration test are empirically decomposed into a set of intrinsic mode functions, from which tag vectors are constructed, then tag angles are defined to diagnose the failure of the board. Combined with the load direction to the structure, the position and the direction of the crack can be identified by the tag angles.