The boron nitride fibers were heated at the range of 600-1400oC in flowing nitrogen and air, respectively, and the effects of heat treatment on the structure, composition and morphology of BN fibers were studied. The results showed that BN fibers exhibited smooth surfaces, and that t-BN was the main phase with a little B2O3 included. After heat treatment at 1400oC in nitrogen atmosphere, the fibers displayed rough surfaces with little change in mass. Better crystallinity was obtained with the increasing temperature. During heat treatment in air, the fibers were oxidized severely as the temperature went up, especially at 1400oC. The volatilization of B2O3, HBO2 and H3BO3 led to the pores on the surfaces of the fibers, while the boron oxide glaze and nitrogen gas produced during the oxidation process protected the fibers from further oxidation. The boron nitride fibers were heated at the range 600-1400oC in flowing nitrogen and air, respectively, and the effects of heat treatment on the structure, composition and morphology of BN fibers were studied. The results showed that BN fibers exhibited smooth surfaces, and that t-BN was the main phase with a little B2O3 included. After heat treatment at 1400oC in nitrogen atmosphere, the fibers displayed rough surfaces with little change in mass. Better crystallinity was obtained with increasing temperature. During heat treatment in air, the fibers were oxidized severely as the temperature went up, especially at 1400oC. The volatilization of B2O3, HBO2 and H3BO3 led to the pores on the surfaces of the fibers, while the boron oxide glaze and nitrogen gas produced during the oxidation process protected the fibers from further oxidation.