In teaching proportion, the teachers usually give the students a formal formula, such as cross-multiplication, without providing them the meaning of proportionality. Thus, the students just apply it in order to solve proportional problems. However, it does not connote that they understand what proportionality means. In this paper, the authors attempt to explore how the informal proportional reasoning can be developed to a more formal level. The development of student's proportional reasoning is described in three levels since the representations of students' reasoning are different. In order to support them, several instructional activities were designed which were based on the heuristic of Realistic Mathematics Education (RME) within a learning trajectory. In this case, design research was chosen as an approach in investigating how the design works in the classroom. The result shows that the designed students' activities could help students learn about proportion and develop their proportional reasoning.