Common duckweed (Lemna minor L.) is ideally suited to test the impact of metals to freshwater plants. Literature on cadmium (Cd) and uranium (U) oxidative responses in L. minor is sparse or, for U, even non-existing. It was hypothesized that both metals impose concentration-dependent oxidative stress and growth retardation on L. minor. Using an OECD described 7-day growth inhibition test the metals adverse impact on L. minor growth was confirmed with EC50 values for Cd and U of, 6.5 ± 0.9 µM and 29.5 ± 1.9 µM, respectively and EC10 values amounted up to 1.5 ± 0.2 µM and 6.5 ± 0.9 µM, respectively. The metal-induced oxidative stress response was compared by assessing the activity of different antioxidative enzymes. In conclusion, although U and Cd induce a similar growth arrest in L. minor, the U-induced oxidative stress responses, studied here for the first time, differ greatly from those of Cd.