Acute toxicity and antioxidant activity of hydroethanolic leaves extract of Solenostemon monostachyus (Esomo) were investigated. The method used for antioxidant evaluation was 2, 2-Diphenyl-1-picrylhydrazyl (DPPH) and the method of Miller & Tainter was employed for the acute toxicity. The lethal dose inducing death for half of tested mice (LD50) for Esomo was 1.9± 0.001 g/kg body weight. According to the classification of Diezi (1989), this drug is weakly toxic. Esomo's concentration at 50% DPPH radical scavenging activity (IC50) was 19.95 ± 0. 1µg/ml. Esomo (100 µg/ml) induced a DPPH scavenging activity of 86.25 %. Similar results were obtained with acid gallic and ascorbic acid applied at the same concentration and they induced a DPPH scavenging activity of 91.66% and 98%, respectively. Our results show that the extract of S. monostachyus exhibits an antioxidant property. The property is due to the presence of phenolic compounds, such as flavonoids, tannins, coumarins, sterols and terpins, shown in the former phytochemical study. The weak toxicity and the significant antioxidant activity of leaves of S. monostachyus could explain its use in folk medicine.