Transition to kindergarten is an important developmental milestone for children and families. This study explored factors associated with kindergarten readiness focusing on the impact of a summer kindergarten transition program. A mixed-method design was employed, and the qualitative data from interviews with 16 randomly selected kindergarten teachers and the quantitative data of assessment scores of 320 kindergarteners were analyzed. Compared with children who were eligible but did not participate in the transition program, those who participated in the program had significantly more family engagement and stronger home-school relations. Smaller class sizes were associated with children's higher readiness scores in both academic achievement and social-emotional development. Larger school enrollment was associated with children's higher readiness scores in academic achievement, more family engagement, and stronger home-school relations. Higher school poverty was associated with children's lower readiness scores in social-emotional development. The findings can be used to inform program development and policymaking.