An 8-week study was conducted to determine folic acid requirement and its effect on antioxidant capacity and immunity in juvenile Chinese mitten crab Eriocheir sinensis (Milne-Edwards, 1853), followed by a challenge assay with the pathogen Aeromonas hydrophila for 2 weeks. Folic acid was added to a basal diet at seven levels (0, 0.5, 1.0, 2.0, 4.0, 8.0, 16.0 mg folic acid kg-1 diet), and a diet free of folic acid and vitamin B12 was also included as a control. Crabs were fed twice daily in 32 tanks with 7.76-8.17 mg oxygen L-1, 25.0-31.0 °C and 7.5-8.3 pH. Growth and feed efficiency were significantly greater in crabs fed ≥2.0 mg folic acid kg-1, but not significantly different between crabs fed diets >2.0 mg folic acid. The superoxide dismutase activity and glutathione S-transferase activity were highest in crabs fed ≥2.0 mg folic acid kg-1, followed by those fed 0.5 and 1.0 mg folic acid kg-1, and the control diet. The malondialdehyde content was highest in crabs fed the control diet, followed by those fed 0 mg folic acid kg-1, and the lowest value occurred in those fed ≥0.5 mg folic acid kg-1. Phenoloxidase activity and total haemocytes were significantly higher in crabs fed ≥2.0 mg folic acid kg-1 than other diets. Crabs fed 2.0 mg folic acid kg-1 had the highest lysozyme, acid phosphatase and alkaline phosphatase activities but the lowest cumulative mortality. The optimum dietary folic acid requirement by E. sinensis was estimated at 2.29-2.90 mg kg-1 diet.