Phyllanthus amarus (PA) is used for treatment of several diseases such as liver disease, hyperlipidemia and diabetes mellitus in Thai traditional medicine and worldwide. Oxidative stress plays a major role in the pathogenesis of degenerative diseases induced by free radicals such as hyperlipidemia, diabetes mellitus and cardiovascular disease. So the effect of PA extract (PAE) on antioxidant and lipid metabolism gene expression in HepG2 cells was studied. We found that PAE possessed high levels of total phenolic contents and total antioxidants within the range 379.3 ± 7.0 gallic acid equivalents mM/kg dry mass and 11,079.2 ± 336.0 Trolox equivalents mM/kg dry mass using Folin Ciocalteu phenol (FCP) assay and oxygen radical absorbance capacity (ORAC) assay, respectively. PAE at high doses (2,000-3,000 mg/L) induced cytotoxicity according to the neutral red assay. However PAE doses of 100- 800 mg/L could reduce intracellular oxidative stress in a dose-dependent manner (P<0.05) using dichlorodihydrofluorescein diacetate (DCFH-DA) assay. PAE significantly enhanced antioxidant activity by induction of GPX1 gene and Nrf2 gene but decreased Nrf1 gene expression, and reduced cholesterol synthesis by induction of LDLR gene and PPARg gene expression using reverse transcription-polymerase chain reaction (RTPCR) assay. It is suggested that PAE may be beneficial for reducing oxidative stress, decrease cholesterol synthesis thereby maintaining overall cellular homeostasis.