Background: Language minority students, who are mostly immigrant students tend to perform at lower levels in school and to be at risk of school failure when they are limited in English proficiency (LEP). Objective: Based on the previous studies that addressed the importance of students' social skills for school success, I examined the social development of the language minority immigrant students from kindergarten to fifth grade and investigated the longitudinal effect of their social skills on their academic performance in comparison with the English-speaking mainstream students. Methods: Using a nationally representative database, the Early Childhood Longitudinal Study-Kindergarten Class of 1998-1999, the language minority student group, which was divided based on their LEP status at kindergarten, was examined and compared with the mainstream student group with respect to their development patterns and levels of social skills from kindergarten to fifth grade. In addition, the longitudinal effect of students' social skills on their reading and math performance was estimated and tested using the two-level hierarchical growth model. Results: Language minority immigrant students from families living in poverty displayed extremely unstable development in all aspects of social skills, and the positive effect of improved social skills was the largest for the group of students who displayed the most unstable social development, which were the language minority immigrant students who did not show LEP at kindergarten and who were living in poverty. Conclusion: This result suggests the needs of students living in poverty, especially language minority students, for relevant supports and intervention.