Isotropic pitch fiber was oxidized with nitric acid vapor and hot air respectively. Carbonization characteristics and mechanical properties of these fibers were investigated by changing the heating rate of carbonization to get the optimum oxidation and carbonization conditions of pitch fibers. Two types of oxidized fibers show quite different weight loss curves during carbonization because of different functiional groups. Nitric acid vapor could reduce and shorten the oxidation temperature and time of pitch fibers com-paring to hot air without lessening the mechanical properties of carbon fibers.