The oxide scale of TP304H austenitic stainless steel was analyzed with SEM, X-ray, and ESA after being oxidized at 650 °C, 750 °C and 850 °C for different time. The results show that: the initial oxide film consists of needle-like cores. With the heating proceeding, the needle-like cores grow up gathering into granular cores, and then new needle-like cores grow up on former granular cores, and gradually grow into flakes which continue to gather into more granular cores then. As the oxide film becomes thicker, this process is in continuous cycle. The subsequent needle-like or flakey cores form on the basis of previous granular cores. The content of metallic elements is analyzed to find the law of diffusion and oxidation. As the figure shows the Fe of the matrix diffuses outward and the oxidation takes place at the oxide film-gas interface, which means a series of reactions does not occur inside the matrix. Below the temperature of 650 °C, the growth of oxide film is relatively slow, while above this temperature diffusion and displacement of metallic elements and oxide film growth rate accelerates. The oxide film is easy to fall off when reaching to a certain thickness, which means that TP304H stainless steel is not suitable in high-temperature and oxidative environment for long service.