Abstract: 5-Fluorouracil-O-β-D-Glttcuronide (Fu-O-G) was a newly synthesized 5-Fu derivative. It was decomposed into 5-Fu by β-Glucuronidase and demonstrated its antitumor effect in tissue. In this previous paper have demonstrated that the antitumor effect of Fu-O-G was greater than 5-Fu. In this investigation, the toxicity of Fu-O-G and 5-Fu on hemopoietic stem cell, immunological function and digestive tract in mice were observed and compared. The test results have shown both the decrease in the number of nucleated cells of femur and formation colony of spleen in mice were caused by giving Fu-O-G and 5-Fu of treated dose. But the inhibition of Fu-O-G was more light than that of 5-Fu. The toxicity on stomach and intestine in mice were found that both drags were the same, using the method of Evan's blue exudation. The immunological function tests have shown that the difference between drugs and controI groups were not found by giving treated dose drugs on cell-immunological function (GVHR), but Fu-O-G and 5-Fu have same inhibition on humor-immunological function.