Primary water stress corrosion cracking (PWSCC) of Ni-base Alloy 600 (Ni-16Cr-8Fe in wt%) has been a major concern in the primary sides of pressurized water reactors (PWRs). In response to the cracking problems in Alloy 600 another solid-solution strengthened Ni-base Alloy 690 (Ni–30Cr–10Fe in wt%) has become the common replacement material for use in PWR service. Alloy 600 and Alloy 690 have an identical crystal structure and similar mechanical properties; however there are noticeable differences in the corrosion resistance and cracking behavior between them owing to their different Cr contents. It is necessary therefore to reveal the root causes of the different cracking behaviors of Alloy 600 and Alloy 690 in PWR primary water to ensure safe service and good performance.PWSCC testing of Alloy 600 and Alloy 690 was conducted using 1/2T compact tension (CT) specimens at 325 ℃. The simulated PWR water was prepared prior to the test in a storage tank. The test conditions were 1200 ppm B (weight) as H3BO3 and 2 ppm Li (weight) as LiOH in pure water a dissolved oxygen content below 5 ppb a hydrogen content of 30 cc/kg H2O and an internal pressure of 15.9 MPa. The crack growth rates (CGRs) were measured depending on the stress intensity factor at a crack tip. Before the CGR test the CT specimens were pre-cracked by fatigue at lengths of 2 mm in air. A surface oxidation test using plate specimens was conducted in the same test conditions as those of the CGR test for a period of 3600 hours. After the tests cracking properties and surface oxidation layers were precisely characterized using SEM high-resolution TEM STEM/EDS and STEM/EELS.The average CGR of Alloy 600 was measured as 7.6 x 10-9 mm/s when the stress intensity factor at a crack tip was maintained at 30 MPa·m1/2 whereas Alloy 690 did not crack under the present conditions. This means that the resistance to PWSCC of Alloy 690 is much higher than that of Alloy 600. From a microscopic examination on crack propagation it was found that the predominant failure mode of Alloy 600 was intergranular (IG) SCC which indicates that the grain boundaries are preferential paths for cracking. On the other hand PWSCC of Alloy 690 was reported to show a mixed mode consisting of IG and transgranular (TG) cracking which indicates that the grain boundaries are not always active for cracking in the case of Alloy 690. It was revealed from a microscopic investigation on the surface that oxygen diffused into the grain boundaries of Alloy 600 from the external primary water resulting in IG oxidation. As a result of IG oxidation Cr oxides formed in the oxidized grain boundaries leaving Ni depletion. The most important finding in Alloy 690 was that the internal oxidation into the bulk grains was promoted resulting in the formation of relatively thick internal oxidation layer whereas the IG oxidation was significantly suppressed owing to the continuous innermost Cr2O3 layer which formed around a grain boundary. The innermost Cr2O3 layer was formed through inward diffusion of oxygen from the surface and grain boundary diffusion of Cr resulting in Cr depletion along the grain boundary. From the present results it is believed that the different IG oxidation behaviors of Alloy 600 and Alloy 690 appear to lead to the different cracking resistance capabilities and cracking behaviors in these alloys.