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A solid-phase extraction (SPE)-gas chromatography (GC)-mass spectrometry (MS) analytical method was developed for the simultaneous analysis of natural free estrogens and their conjugates in wastewater samples. Natural free estrogens and their conjugates in wastewater were successfully separated by the oasis hydrophilic-lipophilic balance solid phase extraction (Oasis HLB SPE) method, and the conjugates were initially enzyme hydrolyzed by beta-glucuronidase or arylsulfatase from Helix pomatia prior to derivatization. N-methyl-N-(tert-butyldimethylsilyl)trifluoroacetamide (MTBSTFA) plus 1% tert-butyldimetheylchlorosilane (TBDMCS) was chosen as the derivatization reagent, and the most appropriate conditions of derivatization were determined to be at 95 degrees C for 90 min. The recovery ratios of nine target chemicals were determined by spiking them in 1 L of ultra-purified water or the influent of a wastewater treatment plant (WWTP). The recovery ratios of six out of nine for the analytes ranged from 73.3 - 114.9% with relative standard deviations (RSD) from 1.6 - 19.9%. The established method was successfully applied to environmental wastewater samples which were collected from one municipal wastewater treatment plant (WWTP) in Osaka, Japan, for the determination of natural free estrogens and their conjugates. In the influent sample, E1, E2, E1-3S, E3-3S, and E1-3G were detected at concentrations of 16.6, 9.6, 8.2, 21.9, and 3.2 ng L(-1), respectively. However, only El was detected at a high concentration of 44 ng L(-1) in the effluent sample, suggesting that it is the dominant natural free estrogen in the effluent.