The aim of this study is to determine 9th grade students' understanding of density in the set of rational numbers. The participants of this study are twenty five 9th grade students who are studying at a high school in the city of Ordu. Data were collected by conducting clinical interviews with the question set which had been presented by Vamvakoussi and Vosniadou (2004). As a result of the research, it was understood that for the majority of students, prior knowledge of the natural numbers was an obstacle in understanding the density of the set of rational numbers. In addition, it was determined that the idea of the numbers in a given interval should be in the same representation with the endpoints of the interval was dominant for the students.