Objectives:The current study examined whether early receptive and expressive vocabulary development is related with children's socioeconomic background by analyzing longitudinal data of Korean children at 3 and 7 years of age. Methods: In general, both vocabulary scores were about 2.5 times higher at 7 years than at 3 years. Socioeconomic background was significantly associated with expressive vocabulary development at age 3, and with the amount of change in the receptive scores between the ages of 3 and 7. While this relationship was maintained in the expressive scores of the 7-year-old, for lower income families the first child tended to develop relatively slowly while the third child developed the quickest. In addition, the results showed that girls' scores tended to be higher than boys' scores at 3-year-old, and the difference was maintained in all other scores. Results: There were effects of household income and birth order which interacted. The birth order effect pattern was very similar to the effect of household income. Conclusion: The results are discussed in comparison with previous Korean findings and with international ones.In any given age group, the language ability of individuals varies (Fenson et al., 1994). The issue of why and how such variability occurs, particularly in early childhood, has been at the center of many studies of language development. This is partly because infants' language ability provides the basic platform upon which subsequent language and academic performance develops (Burchinal, Peisner-Feinberg, Pianta, & Howes, 2002;Hoff, 2013).Longitudinal studies are pivotal to understanding variability in individual ability, as they provide a basis for examining the factors that modulate developmental progress. Through such studies, evidence can be obtained on how children's language abilities and competencies develop in relation to individual resources and con-ditions; the extent to which individual variability emerges early in life; and the trajectories of these abilities in later academic performance and learning. Despite a substantial number of longitudinal, as well as cross-sectional studies, more evidence is needed from a range of cultural, societal and educational settings to identify determining factors.