During the last decade nanomaterials presented a growing interest. Types of nanomaterials were constantly diversified and the range of applications has increased exponentially (1). Multi-Walled Carbon Nanotubes (MWCNTs) are described as several graphene layers overlaid and laminated in a tubular shape. They are used as structural composites: in electronics, car components, etc. Their toxicity was tested on multiple organisms, and contradictory results on different plant species may indicate potential polluting effects and further testing is mandatory (2). The aim of the study is to determine the effects of MWCNTs on processes involved in development and resistance of duckweed plants. Materials and Methods: Lemna minor L., aquatic plant, very common in various areas and known as standard species for toxicity testing (potential bioremediator). MWCNTs: PD15L5-20, produced by NanoLAB Inc., with a 15±5 nm diameter, a length of 5-20 μm and a 95% purity. The experiment was established using testing 3 concentrations of MWCNTs (10, 50, 200 mg/l) in SIS growth medium. For plants anatomy, sections were made using a hand microtome and dyed using Ruthenium-red. Chlorophyll and carotenoid pigments were determined using methods described in (3). Total phenolic content was evaluated as in (4). Mitotic indices and chromosomal aberrations were also investigated. Results: MWCNTs were observed as clusters on the surface of the roots and on the lower epidermis. Also, chlorosis and necrotic formations were observed on the surface of the fronds. Anatomically, the upper epidermis has been observed to be thicker than normal. Similar effects were observed by (5). A non-enzymatic response can be observed from the increased synthesis of phenolic compounds at higher concentration. Also, chlorophyll contents were influenced. The mitotic index was slightly reduced and the percentage of chromosomal aberrations was increased, indicating some potential mutagenic proprieties of the materials. Conclusion: MWCNTs presence in cultivation medium induced toxicity on Lemna plants, expressed on morphological and physiological level, especially at higher concentrations - 50 and 200 mg/L. To determine the exact mechanisms and the extent of the negative effects, more testing is necessary.