Abstract: The original specimen of T91 and specimens aged for 50-1 700 h at 650 ℃ were studied by scanning electron microscopy (SEM) and energy dispersive spectrometer(EDS), and their precipitated phases were analyzed as a key point. Results show that the phases precipitated along grain-boundary were composed of Cr and Mo mainly; the account of precipitated phase firstly decreased, then increased, and decreased again; the average size of precipitated phase firstly increased, then decreased and spheroidized finally; the relationship between area fraction of precipitated phase and aging time fitted well with a quadratic function.