Abstract: In order to control the formation condition of oxidation oligomers of flavanols, epigallocatechin-3-gallate (EGCG) was oxidized in vitro and EGCG oligomeric products (EOP) were determined by HPLC-MS analysis. The influences of temperature, pH value, mass ratio of oxidant to EGCG on the oxidation of EGCG and the formation of EOP have been discussed. HPLC-MS analysis showed that there were mainly 3 oligomers, namely product 1 m/z 911, product 2 m/z 929, product 3 m/z 911. Products 1 and 3 were dehydrodimers of EGCG, product 2 was quinone dimer of EGCG. The study of reaction condition showed that oxidation of EGCG was enhanced with increased temperature. However when temperature reached 40℃, the formation of EOP decreased after 10 min; EGCG oxidation reaction could be accelerated with increase of pH value. However, at pH value 9.0, the formation of EOP decreased after 15 min; EGCG oxidation yield was increased with increase of m(oxidant):m(EGCG). However, the formation of EOP did not have a correlated relationship. The optimum conditions of EOP formation were:temperature 25℃, pH value 7.5, m(oxidant):m(EGCG)1.5:1,oxidation time 15 min.