Among 90 Vietnamese medicinal plant extracts investigated for their antioxidant activity by DPPH assay at various concentrations from $10-100{mu}g/mL$, 67 showed an inhibition rate over 50% at $100{mu}g/mL$; 47 had greater than 50% inhibition at $50{mu}g/mL$; 17 showed over 50% inhibition at $25{mu}g/mL$. 8 extracts which exhibited strong inhibitory activity more than 50% inhibition at $10{mu}g/mL$ were further tested for lipid peroxidation inhibition by TBA assay. They displayed activity with $IC_{50}$ values from 30.6 to $158.9{mu}g/mL$. Until now, this is the first report on antioxidant activity of the female flower of Borassus flabellifer, and the stem of Combretum latifolium, Embelia ribes, Spatholobus parviflorus, and Tetrastigma erubescens. Fractionations of the EtOAc extract prepared from S. parviflorus led to the isolation of protocatechuic acid (1), ferulic acid (2), epicatechin (3), and gallic acid (4). These compounds showed significant DPPH inhibitory activity with $IC_{50}$ values from 6.5 to $23.6{mu}M$.