Carbon source was recovered from waste activated sludgedby alkaline treatment and gamma-ray irradiation. The recovered sludge carbon source was used for the biological denitrification. Gamma-ray irradiation of 20 kGy and alkaline treatment of pH 10 were found as an optimized recovery condition of carbon source. The biological denitrification efficiency off the recovered sludge carbon source was compared to that of the methanol using synthetic and real wastewater.