The aim of this study is to determine the algebraic thinking levels, misconceptions and understanding levels of algebraic expressions of 6th, 7th and 8th grade students. In this research, developmental research methods, which is one of the descriptive studies, were used. This study was carried out with secondary school students (6th, 7th and 8th grades) studying in a public school located in a province of the Eastern Anatolia Region in Turkey. The sample of the study consists of 82 students studying at secondary school in the second semester of the academic year 2020-2021. Algebraic thinking levels determination form (ATLDF), consisting of 8 questions and 4 levels, and interview were used as data collection tools. The test was administered to 33 students from 6th grades, 21 students from 7th grades and 28 students from 8th grades, and then interviews were conducted. From the tests and interviews, it was seen that the students had 30 misconceptions about algebraic expressions, that their algebraic thinking levels were quite low, that there were only 3 students who could reach Level-4 and all of these students were female students. From the results, it was concluded that the algebraic thinking level of the students who have problems in the transition from arithmetic to algebra is low; and therefore, it was suggested that the connection between arithmetic and algebra should be established firmly, and studies should be done in this direction.