This article presents the study of the change in natural vibration frequency, which is determined with measured acceleration data of the structure when there is a weak reduction in a cross-section. Conducting a simulation with the initial structure and damaged structure has been evaluated the change of natural frequency and mode shape. The results can be used as the basic identification of the location and damage level of the structure. Because of reducing cross-section in different structural positions, the specific vibration form will appear suddenly at different positions. Hence, the level and location of damage can be determined based on the changing of the specific vibration patter.