Frame structures have several applications in industries. They are used to carry all types of loadings. Usually catastrophic failure in these structures initiate from small cracks. Catastrophic failure can be prevented by detecting the cracks early and replacing or repairing the cracked members. The change in dynamics and vibration characteristics is one of the consequences of cracks in structures. In this work, detection of surface cracks in frame structures with regards as the change in natural frequencies of the system is studied. The finite element has been used to compute the natural frequencies of cracked structures. Then, according to the difference in natural frequencies of intact and cracked structures the locations and depths of cracks have been determined by the solution of an inverse problem. For the inverse problem the ant colony optimization algorithm has been employed. It is shown that; while, the changes in natural frequencies are good means for crack detection in a separate beam, it is not sufficient for crack detection in a frame structure. It seems that, other characteristic of the system such as changes in natural modes must be considered.