The purpose of this research is to examine the effectiveness of the diagnostic teaching on fraction. The research method is an experiment approach and the lessons of fraction were taught to the fourth grade students of elementary school. The experimental group consisted of 33 students and was instructed by diagnostic teaching and the control group had 34 students and was taught with small discussion groups. The result of the diagnostic teaching is analyzed through comparisons and contrasts between these two groups from three sources: (a)the learning phenomenon in class; (b)performances on a fraction test after instructions were completed; and (c)scores of fraction questions on the school’s midterm. The result of this research reveals that the diagnostic teaching can create cognitive conflicts on students. Through this process, students’ concept can be clarified and corrected, and students’ misconception on fraction can be dispelled effectively. The main research results are reported as follows: 1.Learning phenomenon in class: The diagnostic teaching is beneficial for learning. 2.Performances on the fraction test: The rates of correct answers are generally higher for students with diagnostic teaching than the control group. (a) The students who received diagnostic teaching performed better in the following concepts: the whole fraction concept, unit concept, and equivalent fraction concept. (b)These two groups of students have no significant differences in their performances on the equal-sharing concept and simple fraction concept. 3. Midterm: The rates of correct answers are generally higher for students with diagnostic teaching than the control group. (a) The students with diagnostic teaching performed better in the following concepts: the whole fraction concept and equivalent fraction concept. (b)These two groups of students have no significant differences in their performances on the fraction assortment recognition, unit commutation and simple fraction concept. Meanwhile, according to the research results, this study proposes some suggestions for designing activities of diagnostic teaching, textbooks, instructions, teacher development, and the future research directions.