This research analyzed the influential factors of grade 9 students study habits and school adjustment on academic achievement. More specifically, the patterns among their study habits, school adjustment, and academic achievement were explored. The resultant gender difference was verified, and the significant correlation of the three variables was investigated. Furthermore, relative effects of the students study habits and school adjustment on academic achievement and the resultant gender difference was studied. For the research, the data of Year 6 of Grade 4 Cohort (As of 2015, grade 9) from Korean Children & Youth Panel Survey by National Youth Policy Institute were utilized. A total of 2,378 students were recruited and studied with three scales for study habits, academic achievement by subject, and school adjustment. SPSS Windows 22.0 Program was used to analyze the effects of grade 9 students study habits and school adjustment on academic achievement. The research results are as follows: First, it was revealed that they had well developed study habits, generally showing high scores above average. Study habits, in particular, showed that male students had higher scores than the counterparts. Second, school adjustment was generally high, and it was revealed that male students better adjusted to school than female students. Third, the level of academic achievement was high in general. Fourth, study habits affected the entire grade 9 students academic achievement. Both male and female students revealed that behavioral control had the greatest effects on academic achievement. Accordingly, there was no gender difference in the influential variables of study habits and academic achievement. Fifth, school adjustment affected the entire grade 9 students academic achievement. In particular, learning activities showed the greatest effect on academic achievement. Therefore, this research implies that various school adjustment programs and proper support and guidance for study habits are required at school and home to help improve grade 9 students academic achievement.