Metastasis can occur following surgical resection of solid tumors and metastasis is the main cause of cancer death. The role of anesthetics used during surgery in cancer metastasis and the underlying mechanism remains largely unknown. Here we show that surgical dissection of primary tumors in mice under anesthesia with sevoflurane leads to significantly more lung metastasis than with propofol in both syngeneic murine 4T1 and xenograft human MDA-MB-231 breast cancer models. Sevoflurane increases the level of serum IL-6, which activates STAT3 and the infiltration of CD11b+ myeloid cells into the lung. Interruption of IL-6/JAK/STAT3 pathway by a JAK inhibitor AZD1480 reverses the pro-metastatic effect of sevoflurane and the associated increase of both activated STAT3 and infiltrated CD11b+ cells in 4T1 model. Our study provides the preclinical evidence informing the distinct effects of anesthetics on metastasis of breast cancers through change of cytokines and the tumor microenvironment.