In general, brittle fracture of steel structures is preceded by the formation of a plastically deformed region ahead of a crack which in most cases is too large to be neglected in size compared to the crack length. A recent trend shows that the crack opening displacement (C. O. D.) theory is a powerful approach to fracture mechanics especially for cases having a large scale yielding ahead of crack. Authors have been investigating on the application of the C. O. D. theory toward the analysis of brittle fracture initiation phenomena in relatively complex steel structures, and have found that the theory agrees well for the analysis of brittle fracture of large structures. This paper deals mainly with the displacement behaviors of loaded members having cracks. Experimental data for C. O. D. using deep notch type specimen agrees well with the theoretically calculated values. The effect of preloading upon critical C. O. D. is experimentally investigated and discussed.