The ability to think critically is an important ability that elementary school students must have. The ability to think critically helps students make the right decisions about what to believe and what actions to take in the present or in the future. The ability to think critically can be developed by an open-ended approach to learning mathematics. The open-ended approach uses open-ended problems that have more than one correct answer and various ways of solving. Open-ended problem is expected to motivate students to find solutions independently so that in the process students' critical thinking ability will increase. This study aims to describe the process of developing mathematics learning tools using an open-ended approach for fourth-grade elementary school students to improve critical thinking ability. The learning tools development model consists of 3 phases, namely preliminary research (needs and context analysis, review of literature, development of a conceptual or theoretical framework for the study), prototyping phase (syllabus design, lesson plans, worksheets and expert validation) and assessment phase (limited trials). The trial in this study used a one-group pretest-posttest design with the research subjects of 19 fourth-grade students at Kedungdoro I/306 elementary school. The data collection technique used a device validation sheet, an observation sheet for the implementation of lesson plans and student activities, a student response questionnaire and a critical thinking ability test. The data obtained were analyzed using the average score which was converted according to the defined categories. The results show that the learning tools developed are valid based on the assessment of the two validators, practical based on the implementation of the lesson plan and student activities in the "very good" category, and effective based on the responses of students who are in the "strong" category and student's critical thinking ability that significantly increased with medium category based on paired sample t-test and N-Gain analysis.

Keywords : Development of Learning Tools, Open-Ended, Critical Thinking