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To optimize the natural chemical agents against nuisance phytoplankton, we examined algal removal activity (ARA) of Plant-Mineral Composite (PMC), which already developed by our teams (Kim et al., 2010), on various conditions. The PMC are consisted of extracted-mixtures with indigenous plants (Camellia sinensis, Quercusacutissima and Castanea crenata) and minerals (Loess, Quartz porphyry, and natural zeolite), and characterized by coagulation and floating of low-density suspended solids. A simple extraction process was adopted, such as drying and grinding of raw material, water-extraction by high temperature-sonication and filtering. All tests were performed in 3 L plastic chambers varying conditions; six different oncentrations (0~1.0mL L-1), six light intensities (8~1,400μmol m-2 s-1), three temperatures (10~30???), four pHs (7~10), five water depths (10~50cm), and three different waters dominated by cyanobacteria, diatom, and green algae, respectively. Results indicate that the highest ARA of PMC was seen at 0.05mL L-1 in treatment concentrations, where showed a reduction of more than 80% of control phytoplankton biomass, while 1,400μmol m-2 s-1 in light intensity (>90%), 20~30??? temperature (>60%), 7~9 in pH (>90%), below 50 cm in water depth (>90%), and cyanobacterial dominating waters (>80%), respectively. Over the test, ARA of PMC were more obvious on the algal biomass (chlorophyll-a) than suspended solids, suggesting a selectivity of PMC to particle size or natures. These results suggest that PMC agents can play an important role as natural agents to remove the nuisant algal aggregates or seston of eutrophic lake, where occur cyanobacterial bloom in a shallow shore of lake during warm season