Barium (Bag) ferrite ultra fine powders were synthesized by using sol-gel in which polyethylene glycol 200(PEG200) was used as gelling agent. The transition of Ba ferrite was studied by thermal gravimetric and differential thermal analysis (TG-DTA) technology. The micro structural changes were analyzed using X-ray diffraction (XRD) and atomic force microscopy (AFM) for the specimens annealed at different temperatures. The transition temperatures were 414.55°C and separately corresponding to BaFe2O4 and BaFe,2O19. There were three types of microstructures for Ba ferrite ultrafine powder specimen annealed at 800°C. For the specimens annealed at different temperatures, there were different kinds of Ba ferrites. The ferrite powder consists of BaFet2O19 and BaFe2O4 for the specimen annealed at 800°C, and only BaFe^O^ can be found in the specimen annealed at 1000°C. The magnetic properties, a , and H c of BaFC|2O19 ultrafine powders are different from that of BaFe12O19 bulk material.
