The safety and toxicity of CeO2 nanoparticles (nanoceria) are of growing concern due to their potential applications in biological and medical fields based on the radical scavenging and UV-filtering properties. In this paper, the ultrafine monodisperse (2-5 nm) water-insoluble (CeO2-P) and water-soluble nanoceria modified with various functional groups of dextran (CeO2-dextran), polyacrylic acid (CeO2-PAA) and ethylenediamine (CeO2-EDA) on surface were synthesized via alkaline-based precipitation and inverse microemulsion methods. The cell uptaking, oxidative stress and cytotoxicity of these nanoceria on human gastric cancer cell line (BGC-803) were systematically investigated. It is found that the cell uptaking of nanoceria is largely relied on the function groups on its surfaces and followed the order: CeO2-P > CeO2-EDA > CeO2-dextran > CeO2-PAA. Moreover, the oxidative stress of BGC-803 cells is obviously affected by the antioxidant capacity of nanoceria determined by Ce3+/Ce4+ ratio, which eventually causes the cell viability variable once the nanoceria entered into BGC-803 cells. In addition, the cell viability is also closely correlated with the concentration and surface characteristics of nanoceria. The cytotoxicity of nanoceria on BGC-803 cells is largely dependent on its surface functional groups. Our work may provide guidance on the cytotoxicity of ultrafine monodisperse nanoceria for their uses in biological and medical fields.