Three laboratory-scale experiments were conducted to assess the oxidative stress responses of brackish water clam Corbicula japonica to feeding activity and salinity level. Natural brackish water from Lake Hinuma was used in experiments I and II, while experiment III used artificial brackish water with cultured diatoms as the food source. During experiment I, the oxygen radical absorbance capacity (ORAC) varied greatly when the initial suspended solids (SS) concentration was 50 mg SS·L-1. As a result, no significant difference in ORAC was found between the initial SS concentrations of 5 and 50 mg SS·L-1 (p > 0.05). In contrast, during experiment II, ORAC decreased from 6.4 to 3.5 μmol Trolox Equivalent (TE)·mg protein-1 at the SS concentration of ~5 mg SS·L-1 (p < 0.05). The rate of carbon uptake in experiment I (SS concentration = 5 mg SS·L-1) was ~2.3 times greater than that in experiment II. These results indicate that SS availability has a great effect on ORAC in C. japonica. During experiment III, ORAC increased under initial SS concentrations of 0 and 40 mg SS·L-1 at salinities of 10 (p < 0.01) and 20 psu (p < 0.05), respectively. In contrast, ORAC decreased significantly decreased during the experiment for SS concentration = 80 mg SS·L-1 and salinity = 20 psu (p < 0.01) and for SS concentration = 120 mg SS·L-1 and salinity = 10 or 20 psu (p < 0.01); ATP content also decreased significantly (p < 0.01). A good correlation was found between the change in ATP content and ORAC. Together, the findings suggest that energy availability and salinity level have strong effects on antioxidant capacity in C. japonica.