This research aimed to develop Mathematics instructional instruments by using problem based learning (PBL) on “straight line equation” Learning Material for the eightth grade of Secondary schools well as to know its effect on the students’ matematical reasoning ability. The method used is mixed methods with concurrent triangulation strategy model namely development research method with Thiagarajan model combined with quantitative method. The research subjects were the students of MTsN 5 Jember of Jember regency consisting of 31 students of trial class 31 of experimental class and 31 students of the control class. The instruments of this research are a observation, an test, and an interview. Quantitative method is applied to analyze the difference of student achievement result among two classes are experimental class and control class. while the qualitative method is applied to describe the process and results of problem-based learning development. The product of this study is the problem based learning teaching administrations. The validity of the average value of those teaching adsministrations (lesson plan, the students’ worksheet, THB) is 3,75. The research results showed that: (1) the instructional instruments resulted is valid, the implementa-tion of the instructional instruments is in practical and effective; (2) the significance value (2-tailed) is 0.000 (p < 0.05), which indicated that the application of learning by using PBL had a significant effect on the students’ matematical reasoning ability.