Abstract: The five factors (i. e. labelling temperature, chloramine T quantity, labelling reaction time, concentration and volume of PBS) effecting the labelling of ~(125)I-SOD by chloramine T method were studied with an orthogonal experimental table. Each factor had four kinds of level. The labelled product was separated by chromatographic column of Sephadex G25, then the labelling rate and relative biological activity Were determined. An experiment of sixteen groups was enough to obtain the optimal labelling conditions. On the basis of study on chloramine T method,the three factores (i. e. labelling temperature, lodogen quantity and reaction time) effecting the labelling of ~(125)I-SOD by Iodogen method were also investigated by another orthogonal experimental table, and three kinds of level were adopted for each factor. The optimal conditions were reached after nine groups of experiment have been completed. The final results showed that for 20 μg SOD when the labelling temperature was 15℃,reaction time was 15 minutes, and the lodogen quantity was 50 μg,the labelling rate was 79. 0%, ~(125)I-SOD relative activity was 98.2%, radiochemical purity of ~(125) I-SOD was 95. 7%, specific radioactivity was 2.92 ×10~5 Bq/μg. The Iodogen method is much better than chloramine T method.