Malaria is one of the most serious health problems in many parts of the world, particularly in Africa and Latin America with a high mortality rate. The situation is further complicated by the spread of drug-resistant parasites in many parts where plasmodium falciparum is endemic. A few alternative drugs are under development, necessitating urgent efforts to identify new classes of antimalarial agents. There is therefore a need to find new, effective and affordable remedies for malaria, including those derived from plants. The clinical utility of the Chinese discovery of artemisinin from the herb Artemisia annua has stimulated much interest in traditional plants as potential sources of new antimalarial drugs. In this study, the antimalarial activity of Artemisia diffusa extracts and the fraction which contains sesquiterpene lactones including Tehranolide, on Plasmodium berghei in vivo on the mice model of malaria was investigated. We did our best to carry out the biological tests as well as the phytochemical investigations from the same collection. It demonstrates that crude extracts of Artemisia diffusa inhibit the growth of Plasmodium berghei in vivo in NMRI mice. The microscopic examination of Giemsa stained slides showed a virtual absence of all blood-stage of murine malaria treated with three concentrations of herbal extracts including 27, 2.7 and 0.27 mg/ml. These observations suggest that the active constituents in the extract may be cytotoxic for P. berghei, thereby inhibiting their development to the erythrocytic stage. The results specifically indicated the inhibitory effects of the A.diffusa crude extracts and the fraction which contains sesquiterpene lactones including Tehranolide, on the developmental stages of P. berghei by decreasing parasitaemia.