Abstract: The increasingly extensive production and utilization of engineered nanomaterials (ENMs) has led to release of large quantity of the substances into the aquatic environment, where ENMs may interact with other contaminants, and alter their bioavailability and ecotoxicity. Heavy metal cadmium (Cd) and benthic macroinvertebrate Bellamya aeroginosa were selected as model contaminant and test species, respectively, for an experiment to explore changes in Cd bioaccumulation and effects of Cd on two key biomarkers, superoxide dismutase (SOD) and malondialdehyde (MDA), in the hepatopancreas in the presence of multi-walled carbon nanotubes (MWCNTs), different in tube diameter through sediment toxicity test. Results show that MWCNTs, regardless of tube diameter significantly increased Cd bioaccumulation, and MWCNTs, 8-15 nm in tube diameter, displayed a greater effect promoting Cd bioaccumulation than MWCNTs, 20-30 nm in tube diameter. When Cd was low in concentration (5 μg·g^-1), the presence of MWCNTs did not affect Cd effect on SOD activity; when Cd was medium in concentration (25 μg·g^-1), the presence of MWCNTs significantly induced SOD activity, and the effect was more obvious in the presence of small-diametered MWCNTs than in the presence of big-diametered MWCNTs; and when Cd was high in concentration (100 μg·g^-1), the presence of MWCNTs significantly inhibited SOD activity, with the presence of small-diametered MWCNTs in particular. When Cd was medium in concentration, MDA level was much higher in the treatments with MWCNTs than in CK, however, when Cd was high in concentration, MDA level was significantly lower in the treatment with small-diametered MWCNTs than that in the treatment with big-diametered MWCNTs and in CK, indicating that higher Cd bioaccumulation induced by the presence of small-diametered MWCNTs caused severe damage to cells of the zoobenthos, thus lowering drastically MDA level. Therefore, when Cd was medium or high in concentration, the presence of MWCNTs can significantly enhance the ecotoxicity of Cd on B. aeroginosa, which coincides with the change in Cd bioaccumulation in livers, and the effects of MWCNTs on ecotoxicities of various kinds of contaminants may potentially relate to physicochemical property of MWCNTs, environmental media, and species of the organisms.