Vegetation plays an important role in a constructed wetland system. It cannot only absorb material from aqueous phase, but provide surface and oxygen for the growth of microorganisms and carbon source for denitrification reaction. Furthermore, plants can also provide mechanisms to achieve some physical treatment purposes, such as filtration, sedimentation, sunlight shading, etc. The mechanisms for removal materials from water are quite well understood according to many previous researches, especially in Taiwan area, the plants are also useful for environmental restoration. Therefore, macrophytes used in many constructed wetland systems in Taiwan had been chosen with diversified way so as to restore some rare species a long with water purification. However, after a period of time, many designed species in most of these treating wetlands were replaced by weeds, and possibly affected the treatability of the system on wastewater. Meanwhile, some terrestrial intrusive species, such as Mikania micrantha H. B. K., Ipomoea nil (L.) Roth., etc. are causing serious problems for development of normal terrestrial species. Hence, it is necessary to proceed long-term investigations and experiments on vegetation on the development and change of terrestrial plants as well as the intrusive species in order to understand the environmental factors of the vegetation variation of wastewater treatment efficiencies of constructed wetlands. This research project is a 3-year plan including ecological investigation and treatment experiments on chosen constructed wetland site. The research work will not only continue previous 2-year research work of general ecological investigation, but enhance the investigation on vegetation. The annual works are described as follows: First year: routine investigations on ecological systems and grassland vegetation. Try to develop indexes of predomination, variation, development, etc. by obtained information. Further analyze the species and combination of the creatures observed in the wetland system and compare with the concentrations of pollutants in the water, sediment, plant bodies. The comparisons should be useful for settling up relationship between the ecological species and the pollutants distribution. Finally, the entire ecological environments affected by pollution conditions will be realized. Second year: continue routine investigations on ecological systems and grassland vegetation. All the generally used ecological indexes would be applied to analyze the combination of all the observed species. In this year, macrophytes (hydraulic plants) in the wetland system would be focused to be investigated for comparing with the variation of pollutants in the system. So that the relationship of biological change with pollution conditions should be found and the environmental impact to the biological habitat by human contamination would also be understood. Third year: continue routine investigations on ecological systems and grassland vegetation. Some macrophytes would be introduced to the wetland system so as to understand the relationship between vegetation development and water quality in the wetland system, under conditions without interfering the natural development of plants and operation of the system, the information obtained from three years research work will be summarized and plan to reveal effects of ecological change on treatment capability of constructed wetlands. Furthermore, empirical equations for estimating the relations of ecological indexes and wastewater treatment in constructed wetlands then can be developed.