The Bay Mau Lake inside the Thong Nhat Park in Hanoi is the second largest lake in\rHanoi. The lake serves not just as an equitable tank for the rainwater, as a reservoir for\rintake of wastewater, as a wastewater treatment pond, but also as a recreation area for\rHanoi people and visitors from other provinces. The Bay Mau Lake is polluted at\ralarming level.\rThe author has carried out a study on the water pollution level and the sources of\rpollution. The study revealed that the lake polluted mainly by domestic wastewater\rdischarging from living quarters with total about 800,000 inhabitants. The author\rdemonstrated different measures to reduce the pollution level in the lake such as raising\renvironmental awareness for the public, a simple, reasonable cheap measure, which can\rbe both realistic and effective. Besides, the author proposed to use bio-treatment of the\rwater discharging into the Bay Mau Lake, building of a pumping station, fish farming...\rThe clay-paper fibre mixture is used for the biological treatment of wastewater at the\rinlets. The advantage of this method is that it is a simple, cheap and realistic solution.\rTreatment of the water using clay and paper fibre mixture for filtration and plantation of\raquatic plants as lotus and water lily together will provide oxygen for the aquatic\renvironment of the lake. As results of the study, it is recommended to use both methods,\rnamely raising environmental awareness for the public and biotreatement method using\rclay and paper fibre, to improve water quality of the lake.