Abstract: The biological effluent in a petrochemical wastewater treatment plant (WWTP) was classified by ultrafiltration membrane separation. The material characteristics, composition and amounts of dissolved organic fractions in different molecular weight ranges were investigated by measuring TOC, UV₂₅₄ and three-dimensional fluorescence spectrum. The results showed that organics on the fraction of molecular weight less than 1 kDa had the largest ratio in total organics of the biological effluent in the wastewater treatment plant, and the DOC of this fraction was about 92.7% of the total DOC. The aromatic structure was rich in 5 kDa and 10 kDa molecular weight components. Three-dimensional fluorescence spectrum analysis showed that dissolved organics on the fraction of molecular weight higher than 1 kDa mainly contained protein and humus materials, among which aromatic tryptophan was the dominant protein.