Nitrogen and phosphorus are important limiting nutrients for plant growth and primary production in aquatic ecosystems. In aquatic ecosystems interactions between nutrients and toxicants and influence on present organisms are possible. The aim of this study was to investigate the effect of phosphorus on L. gibba and the influence of the gradient of Nand P molar ratio on the toxicity of copper. The effect of nutrient solution with different molarratio of N:P and copper concentrations on growth of Lemna gibba L. was researched by using standard Lemna sp. Growth Inhibition Test (OECD, 2002). The laboratory test showed inhibition of reproduction of plants caused by increasing of nitrogen and phosphorus ratio in the nutrient solution. The copper inhibits the growth rates of plants,influences on the concentration of photosynthetic pigments and undermines the integrity of the membrane. Thecopper toxicity increased with increasing of the duration of exposure time. The results show that nutrient enrichment enhances the copper tolerance of L. gibba. A culture that has grown in a nutrient solution of ratio N:P=300 showed to be the most sensitive to copper, and themost resistant culture in nutrient solution with N:P=3 ratio. Standard toxicity tests can underestimate copper toxicity when performed on plants cultured in controlled laboratory conditions in comparison to plants grown in natural conditions. Differences in the calculated values of toxic levels of copper, depending on the ratio of N and P indicate the need of testing the copper toxicity depending on concentration and ratio of nutrients in similar environmental conditions.