Background The continuous activation of transcription factors drives many diseases, including tumors, autoimmune disease, neurodegenerative disease, and male infertility. Thus, Blocking the transcriptional activity of these proteins may inhibit disease progression. In this study, we developed a new method to specifically inhibit the activity of the transcription factor STAT3. Methods Fusing the transcriptional inhibitory domain KRAB with STAT3 successfully blocked the transcription activity of STAT3 in cancer cells without affecting its function in the mitochondria and lysosomes. Results the expression of KRAB-STAT3 fusion protein inhibited the growth of tumor cells. Conclusions The KRAB-STAT3 fusion protein provides a novel approach for drug development for the treatment of cancer or autoimmune diseases. Keywords: KRAB, STAT3, Transcription, cancer, Autoimmune diseases, Neurodegenerative diseases, Male infertility