SUMMARY
This Didactical Design Research which motivated by the curiosity of researchers towards the students’ obstacles in communicating mathematical thinking within oral and written. Students’ mathematical communication is one of the demands in curriculum. Communication skill should be the first one for students. This research aimed developed didactical design about students’ mathematical communication by using think-talk-write strategy. This research involved mathematic and language lecturers, university students, primary school teacher, and primary school students. In this research the topic was about comparison at fifth grade with theme life in harmony'. This research resulted in didactical design of mathematical communication which could overcome fifth grade students' learning obstacle in comparison.