In order to understand if a conventional wastewater treatment progress can provide sufficient protective screen of heavy metals, a long term monitoring was conducted for the influent, effluent, and water at different stages of a domestic wastewater treatment plant. As a result, eight heavy metals such as As, Cd, Cr6+, Cu, Hg, Mn, Pb, Zn were detected. For most of the heavy metals about 40% removal could be obtained by the secondary treatment progress. Of the total removal, 10 to 20% was achieved by the primary settling, while 20 to 40% was achieved by the following biological units. Through a series of adsorption experiments using quartz sand, kaolin of known size distribution as inorganic adsorbents and laboratory-cultured activated sludge as organic adsorbent for adsorbing model of the eight heavy metals, it was found that the adsorption of heavy metals by the inorganic adsorbents well followed by the Langmuir isotherm with the saturation capacity proportional to the specific surface area of the solid particles, indicating a single-layer adsorption property. However for the activated sludge, its adsorption isotherm showed a property of multilayer adsorption and the adsorption capacity was one order higher than the inorganic particles. The different behavior of the activated sludge from the organic particles for model heavy metals adsorption explained the mechanism of heavy metals decay in the different stages of the conventional wastewater treatment plant and indicated that the activated sludge in the biological treatment plant unit played the main role for heavy metals adsorption