Abstract: Pre-oxidized polyacrylonitrile (PAN) fibers were used as a precursor and they were in situ transformed into carbon fibers to toughen alumina ceramics in vacuum sintering. The chemical structure and phase of the pre-oxidized PAN fibers were studied by thermogravimetric/differential thermal analysis (TG/DTA) and X-ray diffraction to determine a suitable sintering process of the composites. The effects of pressing ways and pre-oxidized PAN fiber content on the microstructure and properties of the composites were explored. It is found that the exothermic peak around 444~C in the DTA curve and the diffraction peak around 17~in the XRD pattern of the pre-oxidized PAN fibers are due to incompletely oxidized PAN molecules during pre-oxidation, while the endothermic peak at 1073~C and the diffraction peak around 25.5~show the pre-oxidized PAN fibers having begun to transform into carbon fibers during the sintering. The mechanical properties of the composites prepared by hot pressing are superior to those by pressureless sintering. With the pre-oxidized PAN fiber content increasing, the density and microhardness of the composites decline, however the fracture toughness increases first and then decreases. When the volume fraction of the pre-oxidized PAN fibers is 20%, the fracture toughness of the composites is up to 9.39 MPa·m^1/2. It illustrates that the in situ transformation carbon fibers improve the fracture toughness of the composites. The toughening mechanism is mainly fiber pulling-out and deboning.