This research aimed to describe the mathematical concept understanding skill on junior high school students reviewed from self-confidence. This descriptive qualitative research used Miles and Huberman model which included data reduction, data display, and conclusion. The subjects of this research were 6 students from grade VIII A of SMP Negeri 1 Karangpucungwho were taken using purposive sampling. The data were collected using a questionnaire of self-confidence, mathematical concept understanding skill test on the subject matter of circle, interview, and documentation. The results showed that: a.the subjects with high self-confidence had fulfilled indicators of restating the concept they learned, classifying object by paying attention to the qualified requirements from the concept they learned, giving an example and non-example of the concept they learned, presenting the concept to the form of mathematical representation, and applying the concept to solve a problem; b. the subjects with fair self-confidence had fulfilled indicators of restating the concept, classifying object by paying attention to the qualified requirements from the concept, giving example and non-example from the concept, using the procedure from the concept, and applying the concept to solve a problem; c.the subjects with low self-confidence had fulfilled indicators of classifying object by paying attention to the qualified requirements, from the concept, giving example and non-example from the concept, using the procedure from the concept, and applying the concept to solve a problem.