The low temperature crack propagation (LTCP) for thermally treated Alloy 690 (TT Alloy 690) was indicated by C.M. Brown and W.J. Mills based upon their observation about the significant reduction of J1c at 54°C with intergranular cracking in the alloy. However, it is very important to make clear whether this crack initiation from a defect is due to reduction of fracture toughness or just due to a sub-critical crack growth from a defect such as environmentally assisted cracking under dynamic rising load. In order to examine the existence of sub-critical crack growth of Alloy 690 in simulated PWR primary water with hydrogen over-pressure, LTCP rate of TT Alloy 690 under static loading condition was measured by use of a contoured double cantilever beam specimens under constant loading condition at K of 30 to 40 MPa√m both at 50°C in simulated PWR primary water and at room temperature in air for about 2,000 hours. In this study, fracture resistance tests were also performed by use of CT specimens at 50°C in water with low and high dissolved hydrogen. In addition to these experiments, rapid loading rate change tests for TT Alloy 690 were conducted at 50°C in simulated PWR primary water by CT specimens after the fracture resistance tests for 600 and 1,400 hours. According to these test results, it can be concluded that the LTCP is a kind of sub-critical crack growth that is caused by environmental assisted cracking rather than by the reduction of fracture toughness of these alloys at low temperature in PWR water.