Objective To study effects of signal transducer and activator of transcription 3(STAT3)gene on lung adenocarcinoma cells′EMT.Methods We observed morphological changes of the cells and detected the expression of STAT 3 and Twist by western blot after STAT3-siRNA plasmid was transfected into A549 cells. Meanwhile ,we observed invasion and migration ability of A 549 cells using transwell method and wound healing assay .Eventually ,we verified the correlation between STAT 3 and Twist expression in lung adenocarcinoma tissues by immunohistochemical method .Results si-STAT3 significantly down-regulated the expression of STAT 3 in A549 cells,and pronouncedly inhibited migration and invasion of A 549 cells in vitro.Meanwhile,we also found decreased expression of Twist .The expression of STAT3 was positively correlated with the expression of Twist in lung adenocarcinoma tissue specimens and the expression of two genes was reversely correlated with tissue differ -entiation degree .Conclusion The STAT3 participated in the process of EMT by regulating twist expression , which played a vital role in progression of lung adenocarcinoma .