Abstract The aim of this thesis is the chemical investigation of glycosidic compounds with expected biological activities from selected plants (Fagonia indica and Bassia muricata.) grown in Aseer region. The study started with Phytochemical screening of both plants, that showed two plants contain triterpenoids, flavonoid glycosides, and saponins as a major components. Various bio-assays (antioxidant, antiviral and cytotoxicity) for their extracts and fractions were performed. The results of bioassays revealed the activities of extracts and fractions for both plants. The hydrocarbon, sterol and fatty acid contents of the petroleum ether fractions were determined by GCMS. The chromatographic separations affords six compounds. Three metabolites, 3,4- dimethoxytoluene (A), new flavonoid glycoside, 3-O-[α-L-arabinopyranosyl-(1→2)- α-L-arabinopyranosyl)]-3′-Methylquerectin (B) and 3′-Methylquerectin (C) were isolated from Bassia muricata. From Fagonia indica one flavonoid glycoside; 3-O-[α- L-rhamnopyranosyl-(1→6)-β-D-glucopyranosyl]-kaempferol (D) along with a newtriterpenoid saponin; 28-O-[β-D-glucopyranosylester-(1→3)- β-D-glucopyranosyl] oleanolic acid (E) and known quinovic acid-3-O-(α-L-rahmnopyranosyl-)-28-O-β-Dglucopyranosly ester (F) have been isolated. The structure elucidation was performed by nuclear magnetic resonance (NMR) spectroscopic, one and two dimensions (1D and 2D-NMR) and mass spectrometric methods. The bioassays of the available pure compounds indicated that compound (B) exhibited to high antioxidant activity by DPPH and the compounds (A and E) by xanthin-xanthine oxidaze enzyme while the saponin (F) showed interesting antiviral activity. These findings could be important for evaluating the mode of action of these compounds and their derivatives in vivo to be used in the pharmaceutical industry. In addition, this thesis included investigations of radical scavenging activity of 3'- Methyl-quercetin (C) by using density functional theory (DFT) at B3LYP/6-31G* level of theory. The molecular structure and molecular properties of the most stable rotamers have been investigated at the same level of theory. We have shed light on the structure–property relationship. The absorption spectrum has been computed by time dependent density functional theory (TD-DFT) at TD-B3LYP/6-31G* level of theory. Radical scavenging activity has been explained on the basis of bond dissociation enthalpy (BDE) and the adiabatic ionization potential (IP). Two mechanisms have been explained for the radical scavenging processes, i.e., hydrogen atom transfer and one-electron transfer