This study attempted to examine the school effect of private general high schools in Korea. Classifying private general high schools by their tradition, we analyzed the effect of private schools and their tradition on their students' academic achievement, employing Hierarchical Linear Models. Private schools were classified by their tradition into three types: schools founded before liberation(1944 and before); schools founded between liberation and the implementation of high school entrance policy of student assignment by lottery(1945~1973); and schools founded after the implementation of the high school entrance policy(1974 and after). As for data, we used the data set combining ① the raw data of 2003 KEDI educational statistics DB on general high schools, ② the survey data collected for KEDI research "An analytical study on the qualitative level and actual condition of school education: the case of high school in Korea"(Kim et al., 2003) and ③ the scores of joint exam scholastic aptitude test carried out by Seoul Education Office in 2003. The levels of academic achievement in English and mathematics were found to be higher for private school students than for public school students and the longer the tradition of private schools, the higher the level of their students' academic achievement. When individual and school input variables were controlled for, the levels of academic achievement appeared to be higher in private schools founded before liberation than in public schools.