Isothiocyanates (ITCs) extracts were prepared from fresh broccoli. Their antioxidant properties were evaluated by using the in vitro bioassays, including superoxide anion radical (O-2), hydroxyl radical (HO center dot(-)) and 2,2-diphenyl-1-picrylhydracyl (DPPH center dot) radical-scavenging methods, lipid peroxidation assay, and reducing power assay. The ITCs extracts exhibited significant dose-dependent antioxidant activities (P<0.01). Its antioxidant-stability was affected by temperature and storage condition, although it was more stable than vitamin C (Vc). GC-MS identified sulforaphane in the ITCs extracts. The antioxidant activity of sulforaphane was approximately one tenth to one fifth of that of Vc. Sulforaphane was experimented to be one of dominating contributors to antioxidant capacity of ITCs extracts. However, there was no direct antioxidant activity found for benzyl isothiocyanate (BITC) in vitro. Therefore, it could be concluded that the general group (-N=C=-S) was not the essential part for the antioxidant activity of sulforaphane. It is suggested that the ITCs extracts from broccoli can be developed into a promising food additive for healthy food.