The activities of some creek aquatic plants on water purfication were studied. Two aquatic plamts are locally called Hoteiaoi(Eichhornia crassipes) and Hishi(Trapanatans L. Var. bispinosa). The items of measured water quality were water temperature, DO, EC,pH,T-P,T-N and NO-. Water levels in the creeks are kept high during irrigation season, from June to October, for rice crop production. The creek water is used with cycnlic system by pumping up to paddy fields. During the non-irrigation season, from Novemver to March, water level are kept low for upland crop production. Measuring diurnal changes of profiles of each items, the effects of aquatic plants on water qualities of both seasons were compared. Additionally, the effects of creek bed mud on the water quality were measured by using prototype models of creek set in the green house. Hoteiaoi showed strong activity of absorbing nitrogen during the maturing stage, but after the declining stage, creek water quality was degraded because og the decaying of the plant mass. The result of the prototype model showed that the creek water quality was not so much affected by the creek bed mud except in the vicinity of bottom.