Costunolide, an active sesquiterpene lactone, is derived from a number of medicinal plants and it exhibits a broad spectrum of bioactivities such as anti-inflammatory and potential anti-tumor activity. However, its effects on human osteosarcoma and the underlying mechanisms have not yet been evaluated. Here, we found Costunolide inhibits STAT3 transcriptional activity and the expression level of phospho-STAT3 (Tyr-705). Furthermore, we found Costunolide suppresses STAT3 downstream target genes expression. Finally, our data demonstrated Costunolide inhibits osteosarcoma growth and metastasis in vitro and in vivo. Therefore, our studyprovided the first evidence that Costunolide inhibits osteosarcomagrowth and metastasis by impairing STAT3 signal pathway and Costunolide is a potential candidate in osteosarcoma therapy.