This study aimed at developing mathematics instructional materials based on reasoning and problem solving model with open ended problem oriented which valid, practical, and effective for the seventh grade student. In this study student's handbooks, teacher's instructional guide, and lesson plans were developed. Validity, practicality, and effectiveness of instruments were controlled based on expert's validator, teacher's and student's responses, and the result of the try out. Plomp's development procedure was applied to conduct the study which consists of five stages, namely: (1) preliminary investigation, (2) design, (3) realization/construction, (4) test, evaluation, and revision, and (5) implementation. For the purpose of the study, only the four stages were carried out. The implementation stage could not be involved because of the research limitation. The result of this study showed that the developed instructional materials have fulfilled the criteria of validation value, practicality, and effectiveness. The try out result showed that students were highly involved in teaching and learning activities that affected their high achievement in mathematics. Considering the good result of the try out, it is suggested that the mathematics instructional materials based on reasoning and problem solving model with open ended problem oriented should be implemented in teaching mathematics in the seventh grade of junior high school. Moreover, instructional materials that have been successfully developed in this study, can be used as a guidelines for teacher to develop instructional mathematics with the characteristic of the instructional both in term of developing procedures and process to see the quality of the instructional materials.