To study the effects of soybean isoflavone (SI) on lactating performance, immune function, and interleukin-4 (IL-4) expression level in mammary mast cell of late lactating dairy cows, twelve Holstein dairy cows were assigned randomly to four groups (three duplications in each group). Control group and groups A, B and C were feed different levels of SI (0, 10, 20, 30 mg/kg). Mammary mast cells were isolated from cows and cultured with different concentrations of SI (0, 0.25, 0.5, 0.75 mg/mL). The results showed as follows: 1) SI could significantly slow down the decrease of milk yield (P<0.05), and the milk yields in groups A, B and C were 4.28%, 8.40% and 9.15% higher than control group, respectively; 2) milk protein percentage in group B and C was higher than that of the control group (P<0.05); 3) GH and PRL contents in serum and milk were significantly increased by the supplementation of SI (group B and C, P<0.05), SS content decreased (P<0.05); 4) IL-4 experssion level in serum (P<0.01), milk (P<0.01) and breast issue (group C, P<0.05) were significantly increased by the supplementation of SI. 5) IL-4 experssion level of mammary mast cell was significantly increased by the supplementation of SI (0.25 mg/mL, P<0.01). It can be concluded that SI can improve the lactation performance and enhance the immune function of dairy cows through the regulation on immune factors’ expression in mammary glands and the stimulation on IL-4 secretion.［Chinese Journal of Animal Nutrition, 2010，22（6）:1679-1686］