The purpose of this research is to examine the number sense performance of the classroom teacher candidates taking the Mathematics Education I and II courses. Moreover, it investigates whether there is a change in the number sense performance of the teacher candidates following the Mathematics Education I and II courses. Embedded experimental design was used a mixed methods research design. Pretest-posttest weak experimental design was used in the quantitative part of the study, and a case study for the qualitative part. A total of 74 teacher candidates in the third year of the Faculty of Education, Department of Classroom Teaching at a state university participated in the study. As a data collection tool, the 17-question number sense test was used in both quantitative and qualitative part of the research. The quantitative data showed that there is a significant increase in the number sense performance of teacher candidates after the Mathematics Education I and II courses. The qualitative data indicated that prior to the Mathematics Education I and II courses, the teacher candidates considered mathematics as a course in mathematical operations. They generally tended to use routine rules and algorithms in the number sense test, could not fully conceptualize some mathematical rules and phrases they use, and tried to compute instead of using number sense skills. It was found that after the Mathematics Education I and II courses, there was a decrease in the number of teacher candidates computing, whereas an increase in the number that use number sense.