This study aims to describe the positive and significant relationship between self-concept and understanding mathematical concepts of eighth grade students of SMPN 18 Merangin in the 2019/2020 academic year. The study involved two variables, namely the concept of self as an independent variable and understanding of mathematical concepts as the dependent variable. The population in this study was the eighth grade students of SMPN 18 Merangin 2019/2020 Academic Year. The sampling technique uses total sampling. Data collection techniques using a questionnaire (questionnaire) and tests. The instrument used was a statement of self-concept and a matter of understanding mathematical concepts. Data analysis uses Pearson product moment correlation technique which has three conditions, namely random data (random), normal distribution and linear pattern. Therefore, before analyzing the data, an analysis prerequisite test is conducted, namely the normality and linearity test. The results of self-concept data analysis showed 32 students had very positive self-concepts, 23 students had positive self-concepts, and there were no students who had negative or very negative self-concepts. While the level of understanding of students' mathematical concepts obtained by 18 students in the high category, 20 students in the medium category and 17 students in the low category. Overall class VIII students have a very positive self-concept and understanding of mathematical concepts is moderate. From the results of hypothesis testing it can be concluded that there is a positive and significant relationship between self-concept and the understanding of mathematical concepts of eighth grade students of SMPN 18 Merangin 2019/2020 Academic Year. The magnitude of the coefficient of determination is 11.7%, which means that 11.7% of students' understanding of mathematical concepts is determined by the student's self-concept and 88.3% is determined by other factors.