Abstract: Objective: To investigate the expression of STAT3 (signal transducers and activators of transcription 3, STAT3) and its target gene products including VEGF, bcl-2 in human Esophageal squamous cell carcinoma (ESCC) celllines, and to explore whether the STAT3 signaling transduction pathway was constitutively activated in ESCC cell lines. Methods: Immunocytochemistry and Western blotting were used to determine the protein levels of STAT3, p-STAT3 (activated STAT3),VEGF and bcl-2 in two ESCC cell lines. Nuclear proteins from the ESCC cell lines were extracted to evaluate theDNA-binding activity by electrophoretic mobility gel shift assay (EMSA). Results: Immunocytochemical analysis showed that the protein of STAT3, VEGF and bcl-2 were mainly locaLlized in the cytoplasm,while the protein of activated STAT3 (P- STAT3) were mainly expressed in the nuclei. Western blotting revealed that there were almost the same protein levels of STAT3, VEGF, P-STAT3 and bcl-2 com-pared to that of in positive control cell line. The constitutively activated STAT3 signaling transduction pathway was existed in the two different ESCC cell lines, respectively. In addition, the results of EMSA analysis for nuclear protein extract showed high DNA-binding activity of STAT3. Conclusion: The findings demonstrate that STAT3 signaling transduction pathway is constitutively activated in human ESCC cell lines, suggesting that STAT3 may plays a critical role in the carcinogenesis of esophagus.