Acute toxicity on the water extract of Mori Fructus was examined in male and female mice. The water extract of Mori Fructus was orally administered at a dose of 5 mg/kg, 50 mg/kg, 300 mg/kg and 2,000 mg/kg and had been observed for two weeks. No mortality and abnormal clinical signs were shown for the observation period. At the terminal sacrifice, there were no difference in net body weight gain, organ weight and gross pathological findings among the groups treated with different doses of the water extract of Mori Fructus. The results suggested that under the condition employed in this study $LD_{50}$ would be more than 2,000 mg/kg. All the data obtained the experiments lead to the water extract of Mori Fructus should have very low acute toxicity.