The purpose of this study is to investigate the difference between academic achievement in “Selection and Placement Exam for University” (OSS) of students who have high critical thinking dispositions and of students who have low critical thinking disposition and whether this difference change with students’ gender, class levels (high-school graduated /high school student) and major which they studied in high school. The research was conducted with 121 students who were selected randomly and studying to prepare 2009-OSS in the province center of Kdz. Eregli. For determining the academic achievement, scores of students on test exams for OSS in 2008-2009 academic years have been used. To measure the students’ critical thinking dispositions, the “Critical Thinking Dispositions Scale” (CTDS) developed by Akbıyık in 2002 was used and this scale has the Cronbach Alpha reliability coefficient of .87. According to their scores on the CTDS, students were grouped as two sub-groups. Students who have high critical thinking dispositions were included in the first group and students who have low critical thinking dispositions were included in the second group. Whether the difference of average academic achievement scores of students’ on tests for OSS change according to the first and second groups have been examined using by independent “t” test technique. Furthermore, whether or not there is a relationship between the students’ critical thinking dispositions and their academic achievement in the first and second groups have been examined by using Pearson correlation technique. As a result of this study, there is no statistically significant difference between students’ academic achievement and critical thinking dispositions at level of .01 and also students’ critical thinking dispositions do not change with students’ gender, class levels (high-school graduated /high school student) and major which they studied in high school.