This study aims at determining the therapeutic effect of knockdown of signal transducers and activators of transcription3(STAT3) gene expression by short interference RNA(siRNA) on transplanted Lewis lung cancer in mice in vivo. pSilencer 2.1-U6 STAT3 siRNA against STAT3(STAT3 siRNA) was synthesized. Lewis lung cancer cells were inoculated subcutaneously into C57BL/6 mice. Seven days after inoculation, the tumor-bearing mice were randomly divided into 3 groups and received intratumoral injection of (1) vehicle(PBS solution), (2) vector(negative control), and (3) STAT3 siRNA. Tumor volume and weight were calculated. Tumors and the lungs were excised for 21 days after inoculation. Expressions of STAT3, vascular endothelial growth factor(VEGF), and matrix metalloproteinase-2(MMP2) were analyzed by RT-PCR, Western blot, and immunohistochemical staining. HE staining and TUNEL assay were used to confirm the apoptosis of tumors. The synthetic STAT3 siRNA effectively suppressed tumor growth, prevented tumor from pulmonary metastasis, and induced tumor apoptosis in vivo compared with vehicle and vectore in controls. It significantly inhibited STAT3 expression to contribute to downregulation of VEGF and MMP2 expression within tumors in vivo. This study demonstrates that STAT3 siRNA can effectively inhibit the expression of STAT3 gene within tumors, leading to suppression of tumor growth, prevention of cancer from pulmonary metastasis, and enhancing apoptosis of the transplanted Lewis lung cancer in mice in vivo. This study aims at determining the therapeutic effect of knockdown of signal transducers and activators of transcription3 (STAT3) gene expression by short interference RNA (siRNA) on transplanted Lewis lung cancer in mice in vivo. PSilencer 2.1-U6 STAT3 siRNA against STAT3 ) was synthesized. Lewis lung cancer cells were inoculated subcutaneously into C57BL / 6 mice. Seven days after inoculation, the tumor-bearing mice were randomly divided into 3 groups and received intratumoral injection of (1) vehicle (PBS solution), (2) Tumors and the lungs were excised for 21 days after inoculation. Expressions of STAT3, vascular endothelial growth factor (VEGF), and matrix metalloproteinase-2 ( MMP2) were analyzed by RT-PCR, Western blot, and immunohistochemical staining. HE staining and TUNEL assay were used to confirm the apoptosis of tumors. The synthetic STAT3 siRNA effective suppressed tumor growth, p The impacted tumor from vivo compared with vehicle and vectore in controls. It was demonstrated STAT3 expression to contribute to downregulation of VEGF and MMP2 expression within tumors in vivo. This study demonstrates that STAT3 siRNA can effectively inhibit the expression of STAT3 gene within tumors, leading to suppression of tumor growth, prevention of cancer from pulmonary metastasis, and enhancing apoptosis of the transplanted Lewis lung cancer in mice in vivo.