Signal transducer and activator of transcription 3 (STAT3) is an important member of STATs family and is highly expressed in many kinds of cancer. It regulates the tumor growth by influencing the apoptosis of tumor cells and tumor angiogenesis and becomes a new target for tumor therapy. This article reviews the structure of STAT3,the stimulating factors of STAT3,the tumor growth regulating pathway of STAT3,and its potential as a target for tumor therapy.