* This thesis describes the isolation, the characterization and pharmacological activities of phytoconstituents from the Myanmar medicinal plants Streptocaulon tomentosum Wight & Arnott (Asclepiadaceae), Curcuma comosa Roxb. (Zingiberaceae) and Vitis repens Wight & Arm. (Vitaceae). * Triterpenoids, cardenolides, lignanes, and steroidal saponines including three new substances were isolated from the roots of Streptocaulon tomentosum. * Curcuminoids and sesquiterpenoids including nine new sesquiterpenes were characterized from the rhizome of Curcuma comosa * Polyphenols, fatty acids, and lignanes were obtained from the rhizome of Vitis repens. * Four extracts of each plant were tested for their antifungal properties against Cladosporium cucumerinum Ell. & Arth. according to Gottstein et al. (1982). * Six cardenolides isolated from Streptocaulon tomentosum were tested for their antiproliferative activity in vitro against MCF-7 (human breast cancer cell line) and L 929 (mouse fibroblast cell line). Among six cardenolides, 17α-H-periplogenin-3-O-β-D-digitoxoside, and 17α-H-periplogenin-3-O-β-D-cymaroside exhibit significant antiproliferative activity (IC50 values, < 1μM) against MCF-7. Four cardenolides were examined for their cellular viability in the tumor cell and U 937 (human leukemic cell line) at concentrations 100 μM, 10 μM, and 1 μM. All these four cardenolides show the induction of apoptosis at 100 μM and 10 μM in both cell lines.