Incidence of Ovarian Cancer, which has a high mortality rate, has been reported to increase since 2008 in Indonesia. A number of protective factors have been identified, one of which is breastfeeding. Despite this, published literature on the association of breastfeeding and ovarian cancer in Indonesia has not been found. The aim of this study is to determine the effect of breastfeeding on ovarian cancer in general, as well as among pre and post menopause patients. This case control study was done at “Dharmais”Cancer Hospital, involving 71 patients with ovarian cancer as the cases, and 140 cervical cancer patients who served as the controls. Interviews were done during follow-up visits. Patients, with adouble primary cancer,were not included in the study. The study shows that lifetime breastfeeding contributes to a decrease in the risk of ovarian cancer by 55% (OR=0.45, 95%CI 0.23 – 0.91) among all cases; the decrease was 53% (OR=0.47, 95%CI 0.23 – 0.96) for those who had previously given birth. These results were adjusted for level of education and menopause status at the time of diagnosis. Women with a history of breastfeeding have a lower risk of having ovarian cancer. The longer the duration of breastfeeding, the lower the risk of having ovarian cancer. The reduced risk was more than 50% for those with a lifetime breastfeeding of 24 months or more.