Objective: Sleep disorders are among the most common psychiatric and medical conditions. In the present study, the hypnotic effect of Tanacetum parthenium was studied in mice. Materials and Methods: The hydro-alcoholic extract (HAE) of T. parthenium and three fractions of it, namely water fraction (WF), ethyl acetate fraction (EAF), and n-hexane fraction (NHF), were intraperitoneally (ip) administrated to mice 30 min before injection of sodium pentobarbital (30 mg/kg, ip). Then, 30 min after administration of HAE, motor coordination (rota-rod test) was evaluated. Besides, LD50 of HAE was determined and the cytotoxicity of HAE was evaluated in PC12 cells using the MTT assay. Results: HAE 50-200 mg/kg increased the sleeping time. EAF was the only fraction which could prolong the sleep duration and decrease sleep latency. The LD50 value was 4. 8 g/kg. The extract induced no cytotoxic effects in PC12 cell line. Conclusion: The results suggested that T. parthenium potentiates pentobarbital hypnosis without causing toxic effects. Probably, its effects are mediated by the components present in EAF of this plant.