The Stress Corrosion Crack Growth Rate (SCCGR) in heavily cold rolled Thermally Treated (TT) Alloy 690 exposed to simulated PWR primary water at 360 °C increases with increasing cold rolling ratio, but the SCCGR in cold rolled Mill Annealed (MA) Alloy 690 remains very low, regardless of cold rolling ratio. Cavities were detected near GB carbides in heavily cold rolled TT Alloy 690 before the SCC tests. There is a good correlation between the existence of cavities near GB carbides and high SCCGRs in heavily cold rolled TT Alloy 690. The number of cavities increases with increasing cold rolling ratio and is affected by heating in air at 400 or 475 °C for, ~2000, h and by exposure in simulated PWR primary water at 360 °C. However, the cavities were detected not only in the stressed area but also in the stress-free area of the SCC test specimens of heavily cold rolled TT Alloy 690. By contrast, the effect of Ni content on SCCGRs in Ni base (25–30%)-Cr-Fe alloys is not significant for similar amounts of GB carbide precipitation. The high SCCGRs in heavily cold rolled TT Alloy 690 may be caused by a high density of lattice defects, cavities near GB carbides, cracking of M23C6 primary GB carbides, and hydrogen absorption, but there is no possibility of creep damage at the test temperature of 360 °C. More detailed tests will be needed to confirm this hypothesis.