The purpose of this study is to examine the causal effects of after-school programs (ASPs) and private tutoring on Korean secondary school students' academic achievement. The students' data from the Gyeonggi Education Panel Study were used in this study for the actual data analysis. The study attempted to adjust for possible selection bias toward either the ASPs or private tutoring by using the inverse probability weight of the propensity score, which was derived from a seemingly unrelated bivariate probit model. The results demonstrated that ASPs and private tutoring both contributed to increases in Korean secondary school students' academic achievement, even after controlling for the selection biases toward both treatments. The magnitude of the effects of participating in ASPs or private tutoring was found to differ by education level; in middle schools, private tutoring showed a higher impact on academic achievement than did ASPs, whereas ASPs had a greater influence on high schools students' academic achievement than did private tutoring. Moreover, it was also revealed that there was no interaction effect between ASP participation and private tutoring participation in middle schools, but that a negative interaction effect existed in high schools. In addition, the differential effect of region size was calculated in order to examine the effect of ASPs and private tutoring in more depth, and the results demonstrated that the positive effects of ASPs and private tutoring on secondary school students' academic achievement were only significant in urban areas.