Abstract: Biological and chemical processes taking place in water hyacinth pond containing industrial wastewater were investigated, including the reduction of the petro-chemical pollutants, COD, BOD, phenol, ammonium-nitrogen, aromatic hydrocarbon and sulfide, as well as the changes of pH, conductivity and dissolved oxygen, etc. The peak of COD/BOD ratio curve occurred around 12 days during testing period in hyacinth pond, indicating the effective purification ability of the aqutic plant. Under the experimental condition, the degrading process was found basically in conformity with the exponential function: COD=A積-B稤. The mean surficial purificaton rate (Ps) of the static hyacinth pond was 9.1g COD/m2d. The physical, chemical and biological conditions in hyacinth pond, are considered to be suitable for the purification of petrochemical wastewater. Various kinds of microorganisms inhabiting around root-system of hyacinth are of great importance during the purification process.