STAT3 plays very important roles in cell survive, proliferation, differentiation, and transformation. Constitutively activated STAT3 was found in many cancers and tumors, including melanoma, breast cancer, head and neck cancer. In order to reveal the mechanisms of STAT3 signaling regulation, a yeast two hybrid screening using STAT3 as bait was performed in the 7days mouse cDNA library. Among the positive clones, a novel protein with 259 amino acids, named SIPAR (STAT3 interacting protein as a repressor) with a GenBank accession number AY714985, was isolated to interact with STAT3. The interaction between STAT3 and SIPAR was analyzed using yeast two hybrid experiments and the functions of the interaction in STAT3 activities as a major signaling transducer was studied. The data from ? 茁 -Gal activity colony-lift filter assay and liquid assay showed that SIPAR had a strong interaction with STAT3. The Western blot experiment showed SIPAR was expressed as a 28 ku protein and a larger protein and mainly located in nucleus in mammalian cells. In order to investigate the function of SIPAR on STAT3 signal pathway, STAT3 luciferase reporter system and SIPAR expression plasmids were co-transfected into mammalian cells. The data demonstrated that when SIPAR was overexpressed, STAT3 transcription activity was inhibited dramatically. Finally, the effect of SIPAR on development was investigated in zebrafish. When SIPAR mRNA was injected, the zebrafish development was affected seriously with shortened body axis, which was in agreement with the experiment of inhibition of STAT3. The data suggested that SIPAR was a novel negative regulator on STAT3 activities.