Abstract: The strain mode variation rate used to be applied for locating the damage of in-service beam structures. However, the uncertain initial local flexural stiffness of the beam structures is not well considered in most present studies. Therefore, the actual damage and the original uncertainty of the local flexural stiffness cannot be differentiated effectively. Therefore, the application of this index to real beam structures does not give satisfactory results. A local flexural stiffness detection method is proposed in this paper. In this method, the actual local flexural stiffness value of each segment of the beam can be obtained by solving a set of linear equations. Subsequently, the strain mode variation rate can be employed to locate the damage. It is shown that the damage can be located exactly based on the actual initial flexural stiffness parameters of the beam structures.