Many biological functions of curcumin have been reported. As certain bioactivities of curcumin are eliminated by antioxidants, reactive oxygen species generated by curcumin have been suggested as a relevant mechanism. In the present study, effects of different types of antioxidants on stability and bioactivities of curcumin were analyzed. High concentrations (>4mM) of the thiol antioxidants including N-acetylcysteine (NAC), glutathione (GSH) and β-mercaptoethanol accelerated decomposition of curcumin and other curcuminoids; Submillimolar levels (<0.5 mM) of GSH and NAC rather improved their stability. Ascorbic acid or superoxide dismutase also stabilized curcumin regardless of their concentration. Cellular levels and bioactivities of curcumin including cytotoxicity and induction of heme oxygenase-1 were significantly reduced in the presence of 8 mM of GSH and NAC. The effects were enhanced in the presence of submillilmolar GSH and NAC, or non-thiol antioxidants. The present results indicate that antioxidants with reduced thiol group could directly interact with α, β-unsaturated carbonyl moiety of curcuminoids and modulate their stability and bioactivity.