The potential ecotoxicological consequences about semiconductor crystal nanoparticles (NPs) are a growing concern. However, our understanding of the mechanism of toxicity in NPs is very limited, especially under varying environmental conditions such as ultraviolet (UV) light. We performed an in vivo study employing Daphnia magna to evaluate the mechanism involved in toxicity of cadmium selenide/zinc selenide quantum dots (QDs) with two different organic coatings under an environmental level of UV-B light. We used QDs with mercaptopropionic acid (MPA) and tri-n-octylphosphine oxide/gum arabic (GA) and measured their toxicities under an environmental level of UV-B light. Whole-body reactive oxygen species (ROS) generation and mRNA expression level biomarkers, as well as acute toxicity, were measured in D. magna. With UV-B light, both cadmium (Cd) and GA-QD became more toxic in daphnids. The levels of small Cd molecules (<10kDa cutoff) increased for GA-QD under UV-B; however, the observed acute lethal toxicity could not be explained by the measured Cd level. Under UV-B light, both Cd and GA-QD generated more ROS. In addition, the expression pattern of mRNAs specific to Cd exposure was not observed from GA-QD with or without UV-B light. These observations suggest that the phototoxicity of QDs may be explained not only by Cd release from the QD core but also by stability of surface coating characteristics and other potential causes such as ROS generation.