Stress corrosion cracking of structural materials is an important issue in light water reactors such as pressurized water reactors (PWR) and boiling water reactors (BWR). Proper disposition of the crack growth rate in terms of the engineering parameters is crucial for safe and economic long term operation. Threshold values of the stress intensity factor (K) have been used in some crack growth rate disposition guidelines such as JSME S NA1-2004 for austenitic stainless steels in BWR environments and flaw evaluation methodologies such as ASME XI Nonmandatory Appendix O-3230 for nickel-based Alloy 600 in PWR primary water environments. These threshold K values are based on the presently available experimental data that have been obtained mostly at relatively high K values. The threshold conditions for stress corrosion cracking in high temperature water are discussed more broadly for different cracking systems with various key-controlling parameters. The deformation/oxidation mechanism, which has been applied to quantification of the crack growth rate, is used here for analyzing the threshold conditions for stress corrosion cracking resulting from the interaction between the material and the environment under loading conditions.