The use of Student Facilitator Learning Model and Expalining Learning (Learning that develops students' ideas) in schools is very positive. By using the Student Facilitator Learning Model and Expalining Learning (Learning that develops students 'ideas) students' teaching and learning activities are more interesting, lively, so students actively express their opinions. This can be seen in the results of Cycle I obtained data from the results of student achievement in Class VII-B Odd semester of SMP Negeri 1 Rejoso, Nganjuk Regency 2019/2020 academic year, using the Student Facilitator and Expalining Learning Model Learning students) of 32 children obtained an average on the positive aspect of 6.32 (63%), the psychomotor aspect showed a result of 6.38 (63%) and on the affective aspect the average was 6.78 (68%). So the results of learning achievement of 6.38 with a percentage of 63%. This is still below the mastery learning by 70 (70%). Then it is necessary to carry out activities in the second cycle. And in the second cycle shows the results of student achievement in Class VII-B Odd semester of Rejoso State Junior High School, Nganjuk Regency in the academic year 2019/2020, using the Student Facilitator and Expalining Learning Model of Learning from 32 children get an average on the positive aspect of 89.13 (89%), the psychomotor aspect shows results of 80.75 (81%) and on the affective aspect the average is 81.54 (259%). So the results of learning achievement of 261.8 with a percentage of 84%. This is still above the Mastery Learning by 70 (70%). Then there is no need for activities in the next cycle. And declared Complete. As for group assessment, it can be done through tasks that are done together, from this activity all groups collect tasks on time. So it can be concluded that in terms of affective (cooperation and responsibility) 100% successful / complete. Based on these descriptions, the hypothesis proposed in this study. If the Student Facilitator and Expalining Learning Model is used in Mathematics, it can improve learning achievement, not only develop the positive direction, but also the affective direction, it can be accepted. It will be fixed in his memory and will improve learning achievement. Then this Research Activity was declared Successful