The oxidation resistance of isotropic pitch-based carbon fibers are sudied by thermogravimetric analysis,scanning electron microscope and mechanical propefties measure. The change of weight loss,microtextule and mechanical properties on condition of thermostatical oxidation and nonisothermal oxidation are separately mainly discussed.The results during isothermic oxidation at 316℃ showed that the weight loss of isotropic pitch-based carbon fiber increased and the strength, module rapidly decreased with prolongation of time, but the surface of carbon fiber is smoother and has not surface such as etching pits etc. The weight of isotropic pitch-based carbon fiber decreased more rapidly during the experiment of thermo-variable weight loss after 500℃ than before 500℃.