The purpose of my research work was to assess a variety of Northern Thailand plants tissue extracts for antibacterial and anti-platelet aggregation activity. The Minimum Inhibitory Concentration assay method was used to assess antimicrobial activity of plant extracts, while the Zebrafish Platelet Aggregation Assay and the in vitro Whole Human Blood Impedance Aggregation Assay were used to study anti-platelet activity. Forty one plant extracts harvested from the tissues of 26 plants collected from Northern Thailand were assessed. Thirty-four plant extracts were found to have antibacterial activity against the Gram positive bacteria Staphylococcus aureus and/or Bacillus subtilis, while six plant extracts demonstrated activity against the Gram negative bacterium, Escherichia coli. Thirteen plant extracts exhibited anti-platelet aggregation activity better than the positive control. Two crude plant extracts, twigs from Garcinia sp. and twigs from Goniothalamus chilensis were selected for fractionation. Five of the 12 fractions showed anti-platelet activity. Four fractions (two from each plant extract) were selected for further sub-fractionation. Fourteen of 35 sub-fractions were selected for further testing of anti-platelet aggregation activity with 12 sub-fractions demonstrating positive antiplatelet activity. Positive sub-fractions were analyzed using liquid chromatography-mass spectrometry to determine their chemical properties. Three compounds that possessed anti-platelet activity were characterized by nuclear magnetic resonance. The compounds, all isolated from Garcinia sp., were identified as cambogin, isoxanthochymol and guttiferone F.