It was previously demonstrated by the authors that density of water and the density-related physical properties of water are ones of the major governing factors of corrosion of metals in supercritical aqueous solutions. The water density is expected to have significant effects also on stress corrosion cracking (SCC) of metals in supercritical water. In this study, we have looked into cracking behavior of sensitized and non-sensitized stainless steels in water under various pressures and at fixed temperatures above and below the critical point by using SSRT technique, and discussed its correlation with dielectric constant of water. The experimental results have suggested two different cracking mechanisms of 316(L) stainless steels as follows; For sensitized 316 SS - pure water system; (1) Effects of phase state of water and applied pressure, more essentially, physical property of water, were clearly observed. (2) SCC did not occur in the oxygenated 'gas-like', supercritical water at 400°C/25MPa. (3) Cracking occurred at 400°C/30MPa and the cracking severity was more pronounced as applied pressure was increased up to 60MPa at the same temperature. (4) This variation in cracking susceptibility being dependent on pressure was understood from dielectric constant of water. (5) The results give a strong evidence of the dissolution mechanism. For non-sensitized 316L SS - sulfuric acid water system; (1) Non-sensitized 316L SS severely cracked with IG even in a 'gas-like' supercritical water. (2) Dielectric constant did not affect cracking severity. (3) Cracking was more enhanced at higher temperature. (4) The results suggested oxidation cracking.