Pentace burmanica Kurz, Thai medicinal plant, is commonly used for treatment of diarrhea. In this study, the antioxidant activities, total phenolic, non-tannin phenolic and total tannin contents of Pentace burmanica stem bark from 12 different Thailand markets were investigated.A simple and reliable method to determine (+)-catechin and (- )-epicatechin contents was performed by high performance liquid chromatography (HPLC). The ethanolic extract of Pentace burmanica stem bark at the concentration of 100 μg/ml showed high antioxidant activities. The percentage of free radical scavenging activity ranged between 71.56 –80.26% in 2, 2-diphenyl-1-picylhydrazyl (DPPH) assay. Ferric reducing antioxidant power (FRAP) values were 0.09 –0.29 mM FeSO4/100 μg crude extract. The percentages of chelating activity were of 4.24 –12.14 in metal ion chelating assay. The antioxidant activities in betacarotene bleaching assay were of 22.76 –41.06%. The Pentace burmanica extracts contained phenolic, non-tannin phenolic and tannin contents with the range between 35.85 –51.56, 14.08 –40.66, and 10.90 –21.77 μg catechin equivalents /100μg crude extract respectively. Low contents of (+)-catechin were found in the Pentace burmanica extract(< LOQ); whereas the (-)-epicatechin contents were found to be high (10.66-91.55 μg/mg of crude extract). The results demonstrated that greater amount of phenolic contents lead to more potent antioxidant effects and the different sources of Pentace burmanica showed the variation in both antioxidant activities and phenolic contents.