The effect of two kinds of herbicide (Simazine, Thiobencarb) on the stability of small-scale microcosm system was investigated. The microcosm system was composed of microanimals as predator (Cyclidium glaucoma, Lepadella sp., Philodina sp., Aeolosoma hemprichi), algae as producer (Chlorella sp., Tolypothrix sp.) and bacteria as decomposer (Pseudomonas putida, bacillus cereus, Acinetobacter sp., Coryneform bacteria). Simazine affected only the growth of Tolypothrix sp. The number of which was measured for l5days after the addition of the herbicide, decreased to a fortieth with the addition of 0.08mg·l^-1 or more, while Thiobencarb affected the growth of Cyclidium glaucoma from 1.0mg·l^-1 in two species cultivation system and from 2.0mg·l^-1 in the microcosm system. The influence was mitigated by the diversity of constituents in the microcosm. A. hemprich was eliminated and the number of Chlorella sp. decreased to a third at above 0.5mg·l^-1 concentration of Thiobencarb. In the microcosm, Simazine was very stable, however the concentration of Thiobencarb decreased to 30% for l5days after the addition of 1.0mg·l^-1. It was found that the toxicity, the influence and the stability of pesticides in aquatic ecosystem were evaluated by the behavior of the constituent microorganisms and pesticides.