Students suffer from serious misunderstandings about understanding mathematical concepts, which justifies the plethora of research in mathematics education. Schoonfield's approach to mathematics education not only involves problem solving techniques but also reasoning, problem management, and the use of approaches and control strategies. This study, therefore, aimed to determine the effect of control factor on reducing students' misunderstanding of the reasoning and similarity of triangles in Grade 10 of mathematics. This was a quasi-experimental study consisting of test and control groups using pretest and posttest in both groups. The statistical population of this study included all students (n = 120) at Grade 10 of mathematics in Baneh city, in the academic year 2018-2019. The students were sampled through cluster random sampling method. A pretest was performed on two selected classes, and 20 sample students with the highest misunderstanding were selected from each class. Data were collected using two researcher-made tests. Results indicated that the controlling factor from Schoenfield’s viewpoint could effectively reduce students' misunderstanding in reasoning, similarity, proportion, and polygons.
