Vitamin B-6 has a strong antioxidative effect. It would be useful to determine whether vitamin B-6 supplementation had effects on antioxidant capacities in patients with hepatocellular carcinoma (HCC) who had recently undergone tumor resection. Thirty-three HCC patients were randomly assigned to either the placebo (n = 16) group or the vitamin B-6 50 mg/d (n = 17) group for 12 weeks. Plasma pyridoxal 5'-phosphate, homocysteine, indicators of oxidative stress, and antioxidant capacities were measured. Plasma homocysteine in the vitamin B-6 group was significantly decreased at week 12, while the level of trolox equivalent antioxidant capacity (TEAC) was significantly increased at the end of the intervention period. Vitamin B-6 supplementation had a significant reducing effect on the change of plasma homocysteine (β = -2.4, p = 0.02) but not on the change of TEAC level after adjusting for potential confounders. The change of plasma homocysteine was significantly associated with the change of TEAC after adjusting for potential confounders (β = -162.0, p = 0.03). Vitamin B-6 supplementation seemed to mediate antioxidant capacity via reducing plasma homocysteine rather than having a direct antioxidative effect in HCC patients who had recently undergone tumor resection. The clinical trial number is NCT01964001, ClinicalTrials.gov.