Abstract. Constitutive activation of signal transducer and activator of transcription 3 (STAT3) in numerous cancers, including lung cancer, is one of the major mechanisms of tumor progression and metastasis. The authors previously reported that the metastasis suppressor non-metastasis protein 23-H1 (Nm23-H1) negatively regulates STAT3 activity by inhibiting its phosphorylation on Tyr705. Nm23-H1 is a multifunction protein that has three different kinase activities. By transfecting the five mutants that inactivated three different kinase activities respectively into Nm23-H1 deficient lung cancer cell lines, it was identified that Nm23-H1 S44A (Ser44 to Ala) and Nm23-H1 S120G (Ser120 to Gly) mutant forms were unable to suppress STAT3 phosphorylation on Tyr705, resulting in increased expression of fibronectin and matrix metalloproteinase-9. Notably, protein inhibitor of activated STAT3 was also involved in Nm23-H1 S44A -and Nm23-H1 S120G -mediated suppression of STAT3 phosphorylation. The present results indicated that Ser44 and Ser120 sites of Nm23-H1 may be responsible for its biological suppressive effects of STAT3 and tumor metastasis, which may contribute to illuminate the metastasis suppression function of Nm23-H1 in lung cancer.