Objectives: This study investigated the influence of internal and external factors on vocabulary development in Korean monolingual and English-Korean bilingual children, and examined predictors of their vocabulary skills.Methods: A total of 45 children aged from 3-6 years participated in this study, including Korean monolingual children (N= 30), and English-Korean bilingual children (N= 15). Children completed standardized vocabulary tests to measure vocabulary skills. To examine internal and external factors, a nonword repetition (NWR) task was administered, and participants’ mothers completed the Children’s Behavior Questionnaire-Very Short Form, the Parenting Stress Index, Fourth Edition Short Form (PSI-4-SF), the Parental Styles and Dimensions Questionnaire, and the Language Environment Questionnaire.Results: For monolingual children, internal factors were significantly correlated with Korean vocabulary skills, and effortful control was the only predictor. For bilingual children, there was a significant correlation between internal factors and Korean vocabulary skills. Effortful control and Korean NWR were predictors of Korean receptive vocabulary skills, and Korean NWR predicted Korean expressive vocabulary skills. Their English vocabulary skills were significantly associated with internal and external factors. Quantity of mother’s English input and English NWR were predictors of English receptive vocabulary skills, and quantity of mother’s English input, English NWR and quality of Korean input were predictors of English expressive vocabulary skills.Conclusion: The results suggest that when examining vocabulary skills, we should consider the influence of effortful control for monolingual children and the influence of mothers’ language use at home for bilingual children. Furthermore, NWR should be applied as a clinical tool when assessing bilingual children.