AlN fibers were synthesized by carbothermal reduction using Al2O3 fibers as starting material. The N content of AlN fibers synthesized at 1780°C was more than 31%. The morphology and microstructure of AlN fibers were studied with XRD, SEM and TG-DSC. The fibers containing small AlN grains were pipe-shaped, in which the particle size is about 1µ, diameter 4 ~ 6µ, length/diameter ratio 20 ~ 30. The main reaction process is as follows: Firstly, alumina is reduced by carbon to lower-valent aluminum oxide, which further reacts with nitrogen and forms AlN. Secondly, when the alumina fibers are coated by AlN layer, the reaction takes place simultaneously on both interfaces of the AlN layer.