Rosemary (Rosmarinus officinalis L.) is a popular culinary/medicinal herb. Recent studies have shown it has pharmacologic activities for cancer chemoprevention and therapy. This study evaluated the antiproliferation activity of rosemary extract (RE) against human ovarian cancer cells, and whether the extract and its three main active ingredients carnosol (CS), carnosic acid (CA) and rosmarinic acid (RA) can enhance the antiproliferation activity of cisplatin (CDDP). Our study showed that RE has significant antiproliferation activity on human ovarian cancer A2780 and its CDDP resistant daughter cell line A2780CP70, with IC(50) (50% inhibitory concentration) estimated at 1/1000 and 1/400 dilutions respectively. RE enhanced the antiproliferation effect with CDDP on both A2780 and A2780CP70 cells. A2780 cells were consistently more sensitive to CS, CA, and RA than A2780CP70 cells between 2.5 and 20μg/ml. CS and RA also showed synergistic antiproliferation effect with CDDP on A2780 cells at some concentrations. RE treated by ultrafiltration, dialysis, and removal of phenolics lost the antiproliferation activity suggested that the activity resides in phenolics with MW<1000Da. Apoptosis array study of A2780 cells treated with RE showed that the expression of a number of genes regulating apoptosis were modulated by the treatment. This study showed that RE inhibited the proliferation of ovarian cancer cell lines by affecting the cell cycle at multiple phases. It induced apoptosis by modifying the expression of multiple genes regulating apoptosis, and holds potential as an adjunct to cancer chemotherapy.