- This research is aimed to study the chemopreventive properties of some Thai medicinal plants used in Thai traditional medical formulation for cancer therapy, and to purify the active compounds from the medicinal plants. The water extracts of 14 Thai medicinal plants in the oral preparations from the traditional medical formulae of Phra Kru Wimonkunakorn (Laung Phor Sook) from Wat Pak Klong Makhamtoa, Amphur Maung, Chainat Province, a well known medical text for cancer therapy, named Kum-nam, Kum-bok, Khan-thong-pha-ya-bat, Khao-jao, Khao-yen-nuea, Khao-yen-tai, Toei-hom, Thing-thon, Phrik-khi-nu, Plu, Ma-kluea, Ma-feung, Sap-pa-rot and Non-tai-yak, were investigated for the chemopreventive properties by evaluating their antioxidant activities using 2, 2 ׳-azinobis-3-ethylbenzothaizoline-6-sulfonic acid (ABTS) free radical scavenging assay, oxygen radical absorbance capacity (ORAC) assay, superoxide anion scavenging assay and nitric oxide scavenging assay, their anti-inflammatory activities by inhibiting cyclooxygenase-2 enzyme and their induction the activity of heme oxygenase-1. In the four antioxidant activity tests, it was found that antioxidant properties differed significantly among selected medicinal plants in the individual test. Among them, Non-tai-yak (Stemona tuberose), Plu (Piper betle), Thing-thon (Albizia procera) and Khao-yen-tai (Smilax glabra) water extracts possessed moderate to high activities in all of the method used. In the antiinflammatory activity test by inhibiting cyclooxygenase-2 enzyme, it was found that only the iv water extract of Plu was active in this test. In the induction the activity of heme oxygenase-1 test, it was found that all the water extracts were active in this test. Thus, Plu was selected to purify the active compounds responsible for its chemopreventive properties because it possessed moderate to high activities in all of the antioxidant tests, and also active in the induction the activity of heme oxygenase-1 and in the anti-inflammatory activity test by inhibiting cyclooxygenase-2 enzyme. Regarding the purification of an active compound, the water extract of Plu was partitioned with three different organic solvents; hexane, chloroform and ethylacetate, respectively. The chloroform extract of Plu was further purified using the column chromatographic technique and yielded a major active compound; named hydroxychavicol. The structural elucidation of the isolated active compound was done by nuclear magnetic resonance (NMR) spectroscopic technique. It was found that the 1H-NMR and 13C-NMR data of the isolated active compound were correlated well with the 1H-NMR and 13C-NMR data of hydroxychavicol reported previously. Thus, the chemopreventive properties of Plu might be due to a major constituent, hydroxychavicol, which possesses a good activities in antioxidant test and anti-inflammatory activity test by inhibiting cyclooxygenase-2 enzyme.