The embrittlement fracture mechanism caused by microstructural evolution of 17-4 PH stainless steel at long term aging was studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The solution treated specimen consists largely of lath martensite with a small fraction of elongated delta-ferrite. The spherical particles existed a little in the martensite matrix, while no precipitates were present in the delta-ferrite at the solution treated specimen as non-aging. The precipitation of Fe-Cu in the delta-ferrite causes the aged hardening after long term aging accompanied by decreases in elongation and charpy V-notch energy absorption. The increased fraction of brittle fracture on the fractured surface by impact and tensile test reveals that the embrittlement of the 17-4 PH alloys during long term aging is mainly caused by the precipitation hardening in the delta-ferrite matrix.