Abstract: The aim of this study is to find the inducing differen tiation effect of cinnamic acid on HL-60 cell.The HL-60 cells were treated with cinnam ic acid in vitro,and the change of morphology,the proliferation rate,the ability of NBT reduction and the color reaction of peroxidase of the cells were observe d.The results showed that,when HL-60 cells were treated with 1mmol/L and 2.5mm ol/L of cinnamic acid,the HL-60 cell's proliferation was significantly inhibit ed,the cell's morphology had transformed into more mature myeloid cells,the red uction ability of NBT had significantly increased(P<0.01or<0.05),and the c olor reaction of peroxidase of the cells was much weakened.It indicated that the cinnamic acid had the differentiation effect on HL-60 cell.