Dominant fractions of wastewater effluent organic matter (EfOM) were changed from polysaccharides (PS) to polyhydroxyaromatics (PHA), throughout the constructed treatment wetland connected to a wastewater treatment plant, as measured using pyrolysis gas chromatography-mass spectrometry (Py-GC/MS). The changes in the fractions were also identified, with respect to molecular weight (MW) distributions of the effluent organic matters, as measured using high performance size exclusion chromatography equipped with both UV and fluorescence detectors, for aromatic/hydrophobic and protein-like organic substances, respectively; organic matter, with MWs of approximately 2,500 and 20,000 Da, and approximately 38,000 Da, as measured by the UV and fluorescence detectors, respectively, were newly formed after the wetlands, especially for the samples of the Typha wetland in June and August against in December. Thus, with the above two different analyses, the humification type of transformation of EfOM through the treatment wetland, was believed to occur, probably due to biological transformation (from the comparison of results in June and August with those in December). It was anticipated that the humification of EfOM could reduce biodegradable organic portions of wastewater effluents even though total organic carbon levels were not reduced that much after the treatment wetland.