BACKGROUND Worldwide, ovarian cancer has a high mortality rate due to the difficulty in diagnosing early-stage disease and resistance to chemotherapy agents. Costunolide is a plant-derived sesquiterpene lactone with anti-oxidant properties. This study aimed to investigate the effects of costunolide on cell growth, apoptosis, autophagy, the production of reactive oxygen species (ROS), cleaved caspase-3, and cleaved caspase-9 on the multidrug-resistant ovarian cancer cell line, OAW42-A. MATERIAL AND METHODS The MTT assay determined the proliferation rate of OAW42-A multidrug-resistant ovarian cancer cells and the apoptosis rate was determined using propidium iodide (PI) staining. Autophagy was detected by measuring the expression of LC3 II. Fluorescence flow cytometry was used to measure the levels of reactive oxygen species (ROS) and the mitochondrial membrane potential. Protein expression of LC3 II, beclin 1, cleaved caspase-3, and cleaved caspase-9 were measured by Western blot. RESULTS Costunolide treatment inhibited the growth of OAW42-A cells with an IC₅₀ of 25 µM, resulted in apoptotic cell death, increased the expression of Bax, and decreased the expression of Bcl-2. Confocal electron microscopy showed that costunolide induced autophagy in the OAW42-A cells. Western blot showed that costunolide treatment of OAW42-A cells increased the expression of the LC3 II, beclin 1, cleaved caspase-3, and cleaved caspase-9. Costunolide treatment significantly increased the levels of ROS and reduced the OAW42-A cell mitochondrial membrane potential. CONCLUSIONS Costunolide inhibited growth, apoptosis, ROS generation, and was associated with loss of mitochondrial membrane potential of OAW42-A multidrug-resistant ovarian cancer cells.