The phase transformation during pyrite concentrate oxidation under the conditions of the circulating fluidized bed (CFB) roasting process was experimentally studied at temperatures of 600-900 degrees C, O-2 concentrations of 1%-21%, and particle residence times of similar to 5.0 s in a drop tube furnace (DTF). The X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to analyze the phase composition and microstructure of the product samples. For the roasting process with a short residence time, the products of pyrite concentrate oxidation are in a nonequilibrium phase. At higher O-2 concentration and temperature, Fe3O4 is the dominant product phase. When O-2 concentration increases, the micromorphology of sample products tends to be perfectly spherical particles with smooth surface, difficult for the formed Fe3O4 to be oxidized to Fe2O3. Experimental results indicated that, in the CFB roasting, relatively high temperature and low O-2 concentration are needed to control the phase transformation into Fe2O3.