This study aims to design a learning trajectory-based instruction that is used to teach simplification of fractions using realistic mathematic education and flipped classroom. This study is a design research which aims to develop a learning instrument theoretically and empirically. The instruments used are daily notes, learning trajectory, and learning video. The learning trajectory consists of goals and learning assumptions, activities of teacher and students, and students’ flow of thought. Because the data is qualitative, the retrospective analysis will be done using Miles-Huberman method which consists of data collection, data display, data reduction, and conclusion drawing. The result of this study is a trajectory-based instruction using realistic mathematics education and flipped classroom that is used to teach simplification of fractions. The phases of the trajectory-based instruction are as follows: 1) pre-phase where teachers gave rules to be followed by students during class, 2) self-study phase where students learn from the video and solve problems in it before going to the next phase, 3) synchronous learning phase where students, facilitated by teacher solve realistic problems about fraction simplification by WhatsApp and continued by Zoom, 4) transfer phase where students apply their knowledge by doing exercise and small projects.