The purpose of this research is to determine the role of quantitative reasoning in the problem-solving process of the two students who go in sixth grade of secondary school. In this research, in which the qualitative research method is adopted, there were five weeks of instruction carried out with students who go in sixth grade of secondary school in order to determine how the development of quantitative reasoning led to a change in problem-solving ability. Clinical interviews were conducted with students selected as the focus at the beginning and end of the process. Data from this study were also collected from the data of two students who were interviewed clinically. In the analysis of the data, the thematic analysis method was used. As a result of the research, it was observed at the interviews that the quantitative reasoning skills of the two students, who had low quantitative reasoning ability at the preliminary talks, changed at the end of the teaching process. It has been determined that this change is reflected in the problem solving process of the students and that they are also successful in the problems with medium and high difficulty level according to the pre-teaching. In this process, it has been observed that students can solve problems more than one way, choose appropriate strategies for their problem situations and also increase the use of visual representation which has an effective place to do quantitative reasoning.