In Situ transformed carbon fibers/Al₂O₃ ceramic matrix nanocomposites with Cao-MgO-SiO₂ sintering agent were prepared by hot-pressed sintering technology in vacuum. In the sintering process, pre-oxidized polyacrylonitrile fibers (below named as pre-oxidized PAN fibers) were used as the precursors of In Situ transformed carbon fibers. The micro/nanostructure of composites and interface between In Situ transformed carbon fibers and matrix were investigated, as well as the properties of composites. The results showed that the composites could be sintered well at a relatively low temperature of 1650 °C. During the sintering, the precursors, pre-oxidized PAN fibers, were In Situ transformed into carbon fibers, and the In Situ transformed carbon fibers had the graphitelike structure along the fiber axial direction. The carbon atoms arrangement in the surface layer of the fiber was more orderly than the core. A typical diffraction peak of carbon fiber at 26°, which corresponded to the (002) crystal plane, was observed, and the inter-planar spacing was approximately 0.34 nm. The CaO-MgO-SiO₂ sintering agent formed MgAl₂O₄ and CaAl₂Si₂O8 phases in the interface between In Situ transformed carbon fibers and matrix, therefore improving the interface bonding, and thereby modifying the mechanical properties of the composites.