This paper aims at investigating the causal effects of social behaviors on subsequent reading growth in elementary school, using the "Early Childhood Longitudinal Study-Kindergarten" ("ECLS-K") data. The sample was 8,869 subjects who provided longitudinal measures of reading IRT scores from kindergarten (1998-1999) to fifth grades (2003-2004) in the United States. To examine the causal relationship, propensity score methods were used to match higher and lower groups in four social behavior domains such as Approaches to learning, Interpersonal skills, Internalizing problem behavior and Externalizing problem behavior. Results showed that the matched sample achieved sufficient pretreatment balance between the two groups. To examine the effects of social behaviors on the reading growth, multilevel growth model (MGM) was employed. Comparisons of the matched samples showed that children in the high groups of pro-social behavior or in the low groups of problem behavior at kindergarten entrance started with higher reading skills and developed reading achievement faster than those who were not. This study suggests that children's early social behavior is crucial in reading development. Practical implication and direction of future research are also discussed.