Piper betel Linn., which is commonly known as Betel leaf, is consumed for chewing with Betel nut and lime by some people in Asia. Data on the phenolic compounds of this plant is scarce. This study was aimed at investigating the antioxidant activity, total phenolic content, oil-water partition coefficient and yield of various solvents, and extraction time. Methanol, ethanol, acetone and ethyl acetate extract of Betel leaves extracted for 0.5, 1, 3, 4.5, 6, 24 and 48 hrs were examined for radical scavenging ability against 2,2-diphenyl-1-picrylhydrazyl (DPPHë) and 2,2'-azinobis (3-ethylbenzothiazoline- 6-sulfonic acid) cation (ABTS+ë) radicals. Total phenolic content was evaluated according to the Folin-Ciocalteu procedure. The polarity of the plant extract from various solvents was assessed by determining the oil-water partition coefficient by high-performance liquid chromatography (HPLC). Based on the polarity, the ethyl acetate extract, which had less polarity, was suitable for extracting the phenolic compounds from Betel leaves. The extract showed the highest antioxidant activities, total phenolic content and yield. Betel leaf phenolics are found to have less polarity than other phenolic antioxidants due to their high value of oil-water partition coefficient (2.31+0.03). Extraction time from 4.5 to 6 hrs gave the highest antioxidant activity compared with those from other extraction times. Phenolic compounds are in part responsible for antioxidant activities on leaf extracts. Good relationships between phenolic content and radical DPPH scavenging activity (R = 0.945), including phenolics and radical cation ABTS scavenging activity (R = 0.979) of the extracts, were also found. The results indicated that the extraction solvent and time are important for the preparation of the Betel leaf extract for use as a natural antioxidant.