Recently, the fruits of Hylocereus undatus, known as dragon fruit, have received much attention from growers worldwide. However, there is little toxicological information regarding the safety of repeated exposure to these fruits. The present study evaluated the toxicity of H. undatus fruit flesh after acute administration in fish. In the acute toxicity study, Fingerling were not fed during the experimental period. Various concentrations (100, 250, 500, 1000 and 2000mg/L) of the test solutions were prepared from Hylocereus undatus stock solutions. A group of 10 laboratory acclimatized fingerling of a particular species having the same weight, size and age were introduced into each test concentration of Hylocereus undatus. After 24, 48, 72 and 96hrs monitor the mortality of fish. LC50 values were 48, 72 and 96hrs were more than 371.53mg/l, 3.46mg/l and 1.41mg/l observed respectively. The toxicity study which is essential for an adaptation of the traditional medicine was conducted to identify the tolerance limits of Hylocereus undatus fruit extract. According the obtained LD50 (446.68mg/L) value, these findings support the common practice of administration of Hylocereus undatus fruit as a medicinal decoction in herbal medicine.