At JAERI a ductile pipe fracture test program has been conducted as part of a piping integrity and safety research program on LWRs. In the program the ductile pipe fracture behavior of piping under 4-point bending load without internal pressure has been investigated using 3-, 6-, 12-inch diameter stainless steel and 6-inch diameter carbon steel pipes with a through-wall or a part-through circumferential crack in the inner surface of the test pipes. In this paper are described the evaluation of the pipe test data with respect to the validity of net-section stress approach and the flaw acceptance criteria of ASME code Sec.XI, IWB 3640. Pipe fracture data are also used to evaluate the critical conditions at the onset of an unstable ductile pipe fracture under displacement controlled bending load, and a procedure is presented to evaluate the stability of the piping system against unstable ductile fracture using empirical expressions of pipe fracture data.