Species-specific differences in the toxicity of manufactured nanoparticles (MNPs) have been reported, but the underlying mechanisms are unknown. We previously found that CeO₂ NPs inhibited root elongation of head lettuce, whereas no toxic effect was observed on other plants (such as wheat, cucumber and radish). In this study, interactions between Lactuca plants and three types of CeO₂ NPs (lab-synthesized 7 and 25 nm CeO₂ NPs, and a commercial CeO₂ NPs) were investigated. It was found that CeO₂ NPs were toxic to three kinds of Lactuca genus plants and different CeO₂ NPs showed different degrees of toxicity. The results of X-ray absorption near edge fine structure indicate that small parts of CeO₂ NPs were transformed from Ce(IV) to Ce(III) in roots of the plants that were treated with CeO₂ NPs during the seed germination stage. But the high sensitivity of Lactuca plants to the released Ce³⁺ ions caused the species-specific phytotoxicity of CeO₂ NPs. Differences in sizes and zeta potentials among three types of CeO₂ NPs resulted in their different degrees of biotransformation which accounted for the discrepancy in the toxicity to Lactuca plants. This study is among the few, and may indeed the first, that addresses the relation between the physicochemical properties of nanoparticles and its species-specific phytotoxicity.