Online learning is one of the learning systems that can be used as a means in the teaching and learning process that is carried out without direct or face-to-face meetings between educators and students. In online learning, teachers are required to be creative and innovative in applying suitable learning models, one of which is by applying peer tutoring learning models. This study aims to determine the increase in the ability to understand mathematical concepts of students who use peer tutor learning models. This type of research is a quasi-experimental research with a pre-test and post-test control group design. The sample in this study were students of class VIII C and VIII F of SMP Negeri 1 Suranenggala as many as 30 students. The instrument used is a test in the form of 10 description test questions which are arranged based on indicators of understanding the concept of the Pythagorean theorem material. The results showed that there was a significant increase in the ability to understand mathematical concepts of students who were given treatment using a peer tutor learning model than students who were not treated.