This research aimed to produce a learning trajectory that can help students in 8th grade in learning functions. The learning approach used in this study was PMRI approach by using the context of the thermometer. This research used design research through three stages: preparing for the experiment, the design of experiments, and the retrospective analysis. Data collection techniques used were interviews, video footage and photographs, written tests, and field notes. The study involved six students in the 8th grade, which were composed of students with high, medium and low of capability in the first cycle, and involved 20 students in second cycle. The research conducted to produce learning trajectories which consists of a series of learning activity in the process. The results of this study are to produce a learning path consisting of exploring the context of the thermometer, presenting functions, determining (domains, codomains and ranges), finding function formulas and function values, and being able to solve the function problems.