Abstract: The expression vector Stat5 was analysed for the change of effects on lactation ability of dairy cow mammary gland epithelial cells (DCMECs). The expression vector was stably transfected by G418. The proliferations and viabilities of DCMECs, expression of β-casein, lactose, stat5 gene and STAT5 protein were detected by cell counter analyser, HPLC, Western Blotting and Real-time PCR. Compared with the control group, the proliferations and viabilities of DCMECs, expression of stat5 gene and STAT5 protein were both increased （P＜0.01);β-casein, lactose and pSTAT5 protein were increased （P＜0.05). The results showed that, eukaryotic expression vector pcDNA3.1+- stat5-αS1 could be highly expressed in DCMECs, and could raise lactation ability of DCMECs.