The compound inoculation was investigated, and the influences of COD/N ratio and dissolved oxygen on aerobic denitrification in biological aerated filter(BAF) were tested while treating nitrate wastewater from an iron factory. The results show that the efficiency of denitrification was improved, when the concentration of dissolved oxygen was increased from 1.5 mg/L to 4.2 mg/L. When dissolved oxygen value was 3.5 mg/L, the efficiency was the best, it was 95.4%; but the result was opposite when it were increased. When dissolved oxygen value was 8.0 mg/L; the efficiency was still 44.8%. it can be inferred that there were aerobic denitrifer, and oxygen was the electron accepter during aerobic denitrification. The efficiency of aerobic denitrification was improved. When COD/N were 6-7, it can meet the requirement for carbon source during aerobic denitrification, the removal rate of nitrate nitrogen and COD were up to 96%, 85% respectively. Almost no nitrite nitrogen accumulated.