The work in this Thesis has been taken for the isolation and structure determination of bioactive compounds from selected Euphorbia species grown in Aseer region. Therefore, a Phytochemical screening was done for three of Euphorbia plants with the evaluation of their extracts and fractions for antioxidant activities that showed promoting activities. Furthermore, a cytotoxicity against four cancer cell lines and antiviral activities bioassays were performed for the extracts and fragments of E. schimperiana and E. pepuls that shown no activities. The hydrocarbon, sterol and fatty acid contents of the E. schimperiana and E. pepuls were determined for the petroleum ether fractions by GC-MS. Four compounds were isolated from ethyl acetate and n-butanol fractions of E. schimperiana by classical and modern chromatographic tools (column chromatography - high performance liquid chromatography). The structure elucidation of the isolated compounds were determined by spectroscopic techniques (MS - UV - 1D, 2D - NMR). The antioxidant activities of the pure isolated compounds were evaluated using DPPH free radical scavenging activity...