Objectives: The aim of this study was to investigate the effect of breastfeeding on the occurrence of early childhood caries in Korean infants and toddlers. Methods: Data on oral examinations of infants and toddlers of the National Health Insurance Service were analyzed. The study subjects were children who participated in both the first, second, and third oral examinations and the first general health examination in 2008-2017 (n=142,185). Based on the responses to the questionnaire, the subjects were classified into breastfeeding, formula feeding, and mixed feeding groups. The participants were monitored for the development of early childhood caries in three sequential oral examinations. Results: Based on the oral examination results conducted at 54-65 months old, the decayed-filled teeth index of the breastfeeding group was the highest (2.03±3.08), followed by the mixed (1.96±3.03) and the formula feeding groups (1.82±2.91). The Cox proportional hazard regression model including all the variables showed that the risk of developing dental caries was significantly lower in the formula (hazard ratio [HR], 0.85) and mixed feeding groups (HR, 0.91) than in the breastfeeding group. Conclusions: Breastfeeding children have a higher risk of early childhood caries; therefore, oral hygiene education and regular dental check-ups are necessary.