Duckweeds (Lemnaceae) possess good qualitative and quantitative profiles of nutritional components for its use as human food. However, no studies have been conducted on the probable presence or absence of any adverse effects. The extracts from seven duckweed species (Spirodela polyrhiza, Landoltia punctata, Lemna gibba, Lemna minor, Wolffiella hyalina, Wolffia globosa, and Wolffia microscopica) covering all five genera of the plant family were herewith tested for cytotoxic effects on the human cell lines HUVEC, K-562, and HeLa and for anti-proliferative activity on HUVEC and K-562 cell lines. From these assays, it is evident that duckweeds do not possess any detectable anti-proliferative or cytotoxic effects, thus, the high nutritional value is not diminished by such detrimental factors. The present result is a first step to exclude any harmful effects of highly nutritious duckweed for human.