To research the antioxidant chemical mechanism of theaflavin, the antioxidant activities of theaflavin were measured in vitro using several chemical models, including DPPH· (1,1-diphenyl-2-picryl-hydrazyl) scavenging, ABTS⁺· \[2,2’-azino-bis (3-ethylbenzthiazoline-6-sulfonic acid)\] scavenging, and Cu²⁺-reducing power assays, which was compared with the positive control Trolox. In these assays, theaflavin along with positive control Trolox exhibited dose-dependently antioxidant abilities. The relative antioxidant levels of theaflavin vs Trolox (i.e. IC_50,Trolox/IC_{50,theaflavin} value) were calculated as 2.57, 3.13 and 3.74, respectively for DPPH·-scavenging assay, ABTS⁺·-scavenging assay, and Cu²⁺-reducing power assay. In conclusion, theaflavin possessed higher antioxidant levels than Trolox. Its antioxidant mechanisms may include hydrogen atom transfer (HAT) and electron transfer (ET). Through HAT mechanism, theaflavin may be transferred into stable ortho-quinone form.