Two kinds of specimens of 17Cr-4.5Ni precipitation-hardened stainless steel were examined; one was treated in an ammonia decomposition gas atmosphere (nitriding gas) at 1338K for 0.6ks, and another one was no-nitriding. Tensile tests were performed using the above-mentioned specimens. The amount of austenite phase of specimens at various strain rates was quantified by means of X-ray diffraction. Although the specimen treated in nitriding gas exhibits about 50% strain in the tensile test, it does not show transformation-induced plasticity (TRIP) phenomenon. In contrast, the no-nitriding specimen, against the strain caused TRIP. Nitrogen gas in nitriding treatment increases the amount of nitrogen up to 0.03mass%. Most of the nitrogen was found to exist as solute atoms in the specimen. The specimen without TRIP exhibits a tensile strength of 1080MPa and an elongation of 49%. On the other hand, the no-nitriding specimen exhibits a tensile strength of 1170MPa and an elongation of 27%. Therefore, the former showed much higher deformation energy than the latter. This investigation provides evidence that the austenite phase of precipitation-hardened stainless steel is stabilized by nitrogen that is an austenite former, so martensite transformation is inhibited, and thus deformation energy increased.