This study aimed to describe the ability of understanding mathematical concept of students Class VII A in MTsAl-Hidayah Purwokerto. This study was descriptive qualitative. The sampling technique was purposive sampling. The subject was chosen based on Final Test score which included 3 (three) groups; group with high level ofability, group with medium level of ability, and group with low level of ability. The instrument used in this study was test of ability in understanding mathematical concept, interview, and documentation. The result showed that (1) students with high level of ability were able to restate a concept, were able to give example and wrong example, were able to serve a concept in any forms of mathematical representative, were able to use, utilize, or choose particular operational procedure, and also were able to apply concept or algorithm in problem solving, (2) students with medium level of ability were fairly able to restate a concept, were fairly able to give example and wrong example, were fairly able to serve a concept in any forms of mathematical representative, were fairly able to use, utilize, or choose particular operational procedure, and also were fairly able to apply concept or algorithm in problem solving, (3) students with low level of ability were not able to restate a concept, were not able to give example and wrong example, were not able to serve a concept in any forms of mathematical representative, were not able to use, utilize, or choose particular operational procedure, and also were not able to apply concept or algorithm in problem solving.