The purpose of this research was to: 1) development of learning devices based guided discovery model in improving of understanding concept and critical thinking mathematically ability of students at Islamic Junior High School; 2) describe improvement understanding concept and critical thinking mathematically ability of students at MTs by using learning devices based guided discovery model; 3) describe students' responses toward learning devices based guided discovery model in improving of understanding concept and critical thinking mathematically ability of students at MTs; and 4) describe the settlement process of the student's answers in solving problems of understanding concepts and critical thinking mathematically ability of students. This research was a research development, it was conducted in two stages, the first stage is the development of learning devices based guided discovery with the reference Four-D model, and the second stage is to try-out of learning package. The populations of this research were all of the students at Private MTs of Medan and the sample chosen is purposive sampling which were in eighth grade A of Private MTs IRA and eighth grade A of MTs Lab. IKIP Al- Washliyah. From the results of trials I and trials II obtained: 1) learning devices that meet the criteria of effectiveness, effectiveness in terms of a) students mastery learning in the classically; b) achievement of learning purpose; and c) learning time; 2) learning devices based guided discovery model is able to improve the understanding concept and critical thinking mathematically ability of students; 3) Students' responses to components of learning devices and learning activities were positive 4) the settlement process of the students' answers to problems solving about the understanding concept and critical thinking mathematically ability of students with guided discovery model more varied and better. Furthermore, it was suggested that teachers can use guided discovery learning model by presenting problems related to daily life as an alternative student learning.