In order to maintain structural safety, it is important to check whether there is a flaw or not and to measure the flaw sizes with non-destructive inspection. Traditionally radiographic test have been widely used, but it is almost impossible to measure the flaw height and also it can be harmful for human health. Thanks to recent advances in computer technology, instead of radiographic tests, as a new non-destructive inspecting methods, automatic ultrasonic flaw detecting tests are receiving great attention. From one side, the safety assessment of structures can be done in cases where the position and sizes of the flaws existing in structures are well known, but it is a fact that in order to obtain the flaw information, one must depend on non-destructive inspection. So the recognition becomes higher that a more accurate assessment should be demanded with the connection between non-destructive inspec-tion and the fracture mechanics. With such back ground, the purpose of this research is to propose the fracture strength assessment method of cracked structures by automatic ultrasonic testing.