The study aims to investigate middle-grader's understanding of sub-concepts in fraction, including equal-sharing, simple fraction, unit, equal-quantity and equivalence fraction in northern Taiwan. The research methods are paper-and-pencil test and one-to-one semi-structural interviews. The result shows that the students applied their knowledge of integer to deal with the problems of fraction and they regarded numerator and denominator as separate numbers. This phenomenon was evident in students' problem solving strategies. When judging the problems of equal-sharing, they only noticed the number of parts but neglected to judge if each part was equal. Students were confused by the unit terms of unit and content. Middle-graders tended to take the whole content as unit. When the remainder needed to be divided, students added or deducted the content to make the content divisible by integer. Many students compared numerator and denominator separately instead of comparing them as an amount when comparing fractions. Besides, the research also found that the connection between half and one half was not held very solidly by middle-graders. This research will bring up some suggestions for the material, teaching, evaluation and future research on fraction teaching in elementary school.