Herbal medicine has been traditionally used in Asian countries for a long time. Many pharmacological effects are identified in the herbs and these herbs are believed to be safe for human. However, the safety or adverse effect of some traditional herbal medicines has not been established. We have chosen Cyperi rhizoma based on the Korea Herbal Pharmacopoeia and which have been widely used for an anti-inflammatory effect in Korea. The object of the study was to evaluate safety of Cyperi rhizoma in rats. The aqueous extract of Cyperi rhizoma was prepared according to the standard hot water extraction method of the Korea Pharmacopoeia. In the sub-chronic study, the aqueous extract of Cyperi rhizoma was orally administered once daily as 0, 125, 250, 500, 1000 and 2000 mg/kg/day to male and female F344 rats for 13 weeks. There were no treatment related abnormalities in mortality, clinical signs, food consumption, ophthalmologic examination, hematology, serum chemistry, urinalysis, gross observation, organ weight and histopathologic examination. In conclusion, The NOAEL (No Observed Adverse Effect Level) for Cyperi rhizoma aqueous extract was determined as more than 2000 mg/kg/day in the present experimental condition.