Metastasis is the main reason for high recurrence and poor survival of hepatocellular carcinoma (HCC).The molecular mechanism underlying HCC metastasis remains unclear.In this study,we found that argininosuccinate synthase 1 (ASS1) expression was significantly decreased and down-regulation of ASS1 was closely correlated with poor prognosis in HCC patients.DNA methylation led to the down-regulation of ASS1 in HCC.Stable silencing of ASS1 promoted migration and invasion of HCC cells,whereas overexpression of ASS1-inhibited metastasis of HCC cells in vivo and in vitro.We also revealed that ASS1-knockdown increased the phosphorylation level of s727STAT3,which contributed to HCC metastasis by up-regulation of inhibitor of differentiation 1 (ID1).These findings indicate that ASS1 inhibits HCC metastasis and may serve as a target for HCC diagnosis and treatment.