The purpose of this study is to understand the difficulties third grade students have with fractions. Previous research indicates that students have difficulties understanding fractions, and fractions as a number seem to be the most difficult, while fractions as part of whole seem less difficult. Further, research also shows that fractions as numbers are the most important for student understanding of advanced levels of mathematics, such as algebra. This study is based on diagnostic tests using the diagnostic tool “Diamantdiagnosen” and follow-up interviews of 6 students. The diagnostic tool is available on the Skolverkets website and serves as a support for teachers to do a formative assessment of students. The diagnostic test was followed by interviews where the students could elaborate on the difficulties they encountered and how they felt when solving the assignments. In order to categorize the difficulties the students encountered during the diagnostic part, theory of formative assessment, variation and perception of number skills was used. The result of the diagnostic test showed that the students made the most errors with fractions as part of a whole, contrary to previous research.  However, the interviews showed a different result where the students felt that the part of diagnosis with fractions as a number was the most difficult. Comparing the result of the diagnostic test in this study with previous research, the results do not come to the same conclusion. If we look at the views expressed in the interviews, and the assistance strategies (drawing), the students needed help with fractions as numbers in the diagnostic test, and the result pointing in the same direction as previous research. Research conducted in a different setting with older students and slightly more advanced tasks.