The J-R curve, fatigue crack growth rate and characteristics of ductile unstable fracture under monotonic and cyclic load were investigated using 1TCT test specimens which were cut out from STS 42 carbon steel for piping. All the tests were carried out at 100°C. The main results obtained are as follows. (1) The J-R curve under the cyclic load is not a material constant but is dependent on the test conditions. (2) da/dN from typical fatigue data cannot be extrapolated byΔJ if the value of da/dN is above 3×10^-5 mm/cycles. However, it can be extrapolated by using the following equation in which J_max is used. da/dN = C{√(ΔJ)/(B - √(J_max)}^m (3) The J values at instability obtained from the ductile unstable fracture test carred out under the cyclic load of stress ratios R = 0.1 and -1.0 were compared with those from the monotonic increasing load. Both J values at instability were about the same for the loading histories used and the system compliances chosen.