Considerable studies have shown the protective effects of breastfeeding on internalizing problem behaviours in early childhood, yet little is known about the groundwork for this relationship. This study attempted to test the hypothesis that improvement of mothers' positive parenting behaviours by breastfeeding can explain the relationship. We used longitudinal data sets containing a large sample of children from the Panel Study on Korean Children (PSKC). We found that when breastfeeding durations were longer, mothers exhibited higher levels of social parenting behaviours during their children's infancy. Also, mothers' social parenting behaviours during their children's infancy played a protective role in reducing internalizing problem behaviours of children at age 5. In our model, mother's social parenting behaviours partially mediated the relationship between breastfeeding duration and pre-schoolers' internalizing problem behaviours. Results of this study suggest that prolonged breastfeeding should be encouraged for children's mental health and mothers' positive parenting behaviours.