The study explored the longitudinal effects of all-day kindergarten program on the academic performance of students from diverse racial backgrounds and social class from kindergarten to the end of first grade. The study used three measures of reading and math scores from a nationally representative database of the USA, the Early Childhood Longitudinal Study (ECLS). A series of longitudinal multilevel models with various specifications were estimated. The results indicated that all-day kindergarten students began with significantly higher reading scores compared with half-day kindergarten students but there was no significant difference in the growth pattern. Importantly, students from low SES families and Hispanic background displayed enhanced reading achievement in all-day kindergarten.