Purpose: Hizikia fusiforme is renowned for the possession of anti-inflammatory and anti-oxidant properties. In this study, the role of the ethyl alcohol extract of H. fusiforme (EAHF) Results: EAHF suppressed the proliferation of U937 cells in a dose-dependent manner. This effect was closely related to the induction of apoptosis via the downregulation of IAP family members such as IAP-1, IAP-2 and XIAP, as well as Bcl-2 proteins. The results also showed that caspases play an essential role in EAHF-induced apoptosis by generating of reactive oxygen species (ROS). In addition, ROS scavenging by N-acetyl-L-cysteine (NAC) and glutathione (GSH) decreased EAHF-induced apoptosis via the suppression of caspase activity. Although EAHF induced the phosphorylation of mitogenactivated protein kinases (MAPKs), treatment with MAPK inhibitors did not affect EAHF-induced apoptosis. Conclusion: These results suggest that EAHF induces apoptosis in U937 cells via ROS-dependent caspase activation.