This research is aiming to: 1) describe the design of Hypothetical Learning Trajectory (HLT) using realistic context-based learning on linear equations topic; and 2) to describe the improvement of critical thinking ability of students using realistic context-based learning. The topic is limited to the gradient. Design research is used as a research approach. It consists of preparation and design, teaching experiment, and retrospective analysis. The population of this research is students of 8th grade of Junior High School 1. The sample of this research is the 8 B class. We used purposive sampling to determine the sample of this research.

The technique of collecting data which used by researcher are observation, interview, field note, documentation, test, and activity sheets. Researcher interviewed teacher and some students. Interview with students done by choosing their answer which need more explanation. The data of this study is divided into qualitative data and quantitative data. Qualitative data is analysed by using Miles and Hubberman technique. Quantitative data is analysed by using N-gain score.

The results of this research: 1) realistic context-based learning can be used to learn the slope conceptualizations namely highway for physical property concept, signed road whether uphill or downhill and types of staircases for geometric ratio, tables and graphs for algebraic ratio; 2) applying realistic context-based learning can improve critical thinking ability with the average of identification indicator is 3, interpretation indicator is 2, and analysis indicator is 2.