AbstractIt is very important to identify and correct the mistakes that students make about mathematical concepts. The purpose of this study is to detect the secondary school students’ misconceptions about functions in the 2009-2010 academic year .Research data was collected 61 students, included 9th,10th and 11th grade students selected randomly, in Erzincan (2009-2010) academic year. Data collection tools consist of knowledge test and interviews. Frequency tables and percentages were used by analyzing students’ explanations as answers to questions on the function information test. Pearson correlation was chosen to state the meaningful difference among the questions, if there is any, on the Function information test also included four different categories. Student, being believed to be in error, were interviewed with the help of data obtained from Function information test. As a result of the interview, it is most commonly observed that students make mistakes to understand whether a graphic is a function graphic or not, and about the demonstration of the table and function especially on the function information test.