The purpose of this study was to examine the effect of a self-efficacy intervention on primiparous mothers’ breastfeeding behaviors. Participants were recruited from an antenatal clinic at a university-affiliated hospital. Seventy-five primiparous mothers were recruited from November 2013 to February 2014 for the control group, and 75 primiparous mothers were recruited from March to June 2014 for the intervention group. The intervention group participated in a 1-hr prenatal breastfeeding workshop and a 1-hr breastfeeding counseling session within 24 hr after delivery. The Breastfeeding Self-Efficacy Scale–Short Form and the infant feeding method were assessed at hospital discharge, as well as 4 and 8 weeks postpartum. The breastfeeding support program was found to be effective and beneficial to mothers. Nurses should incorporate breastfeeding self-efficacy interventions into their routine care to support new mothers and to increase their breastfeeding self-efficacy and the duration of their breastfeeding exclusivity.