This study used the FACES 2009 cohort to examine the effect of classroom language diversity on the social-emotional development (defined as social skills, approaches to learning, and behavior problems) of the ELL and non-ELL children. A three-level hierarchical linear modeling in which time was nested within the child and the child was nested within the classroom was conducted. The children's ELL status was determined based on their home language and English proficiency. Results showed that the non-ELL children demonstrated a decline in the developmental trajectory of social skills in the classrooms with high language diversity. On the other hand, the ELL children with English proficiency demonstrated a low trajectory of social skills in the classrooms with low language diversity. These findings suggest that the effect of classroom language diversity on the children's social-emotional development depends on the children's language status. Implications and future research are discussed.