The concept of fractions is necessary and vital knowledge in mathematics and has an essential role in perceptions of other mathematical areas. But many claims that this concept is both hard to teach and difficult to understand and thus avoids teaching and using fractions. To find out how the pupils perceive and reason about fractions, didactic theory, George Pólya's, and Frank Lester's problem-solving strategies and the subject theory has been used as a theoretical framework. In the study, which is a case study, participated 24 pupils the ages were between 9 and 10 and performed three diagnoses with a specific focus at part of a whole and part of a number. The diagnostic results were analyzed quantitatively, and then semi-structured interviews were conducted with six pupils according to the selection has obtained from the percentage of the solution frequencies. The results from the diagnoses show that the pupils' solution frequency on the diagnoses was between 14% and 90%. The analyzes from the semi-structured interviews show that the pupils with a lower solution frequency had inadequate knowledge of the concept of fractions and insufficient knowledge of the subject theory and used problem-solving strategies to a very meager extent. The pupils with a higher solution frequency had good perceptions of fractions and had good knowledge of the theory of the subject, and use problem-solving strategies to a much greater extent.