4- (N-maleimide)-2,2,6,6-tetramethylpiperidine-1-oxyl (Mal-Tempol) and bovine hemoglobin (Hb) were used as raw materials and the Hemoglobin site-specifically modified on thiol group with nitrogen oxygen free radicals was synthesized. The modification degree of TEMPO was determined by spectrophotometric method. Moreover, the autoxidation of Hb and Mal-Tempol-Hb was discussed with or without the presence of sodium azide (NaN3).The experimental results demonstrated that the autoxidation rate of Mal-Tempol-Hb was much lower than Hb either with or without sodium azide, and the modification effectively improved the antioxidant capacity of Hb.