This research focuses on critical thinking, which is one of contemporary approaches that emerged in 2004–2005 with revived curricula. In this research, the critical thinking dispositions of secondary students were investigated. The descriptive survey model was used in this research. The research sample is made up of 303 8th grade students receiving education in 2018-2019 academic year at three secondary schools in Adana. The Critical Thinking Disposition (CTD) Scale developed by Ricketts & Ruds (2005) and adapted into Turkish by Demircioğlu (2012) was applied to students selected with unbiased cluster sampling method. The arithmetic mean, standard deviation, p and t values of scores reached from Critical Thinking Disposition (CTD) Scale was calculated. Independent groups t-test to determine if differences are significant or not and Pearson Product-Moment Correlation Analysis was used for evaluating the relationship between variables. Critical thinking dispositions by gender are not significant in Cognitive Maturity, Innovativeness and scale total, however they are significant in Engagement dimension in favor of female students. While students' critical thinking dispositions are not significant in Social Studies, they are .01 in Turkish, .00 in Mathematics, .01 in Science, .02 in Foreign Language, .00 in Religious Culture and Moral Knowledge, and .01 in General Average. In consequence of Pearson Product Moment Correlation analysis, a positive significant relationship was found between scores of students in the CTD scale and scores of Turkish, Mathematics, Science, Social Studies, Foreign Language, Religious Culture and Moral Knowledge and their general achievement averages. So, it has been deduced that gender is not a factor affecting critical thinking disposition; however, there is a significant relationship between critical thinking disposition and academic achievement. It has been suggested to conduct research on critical thinking at secondary school level and to include activities that increase critical thinking in classroom activities.