This study describes students’ conceptual understanding in solving mathematical problems. Conceptual understanding is an important component in learning mathematics. By learning mathematics students are able to apply concepts and think logically. The qualitative research took students of SMP Negeri 1 Gatak Sukoharjo Indonesia. There were 59 students who were tested to find out the conceptual understanding. All students are categorized into 16 students with high mathematical ability (Subject S-1), 25 students with moderate ability (Subject S-2), and 18 students with low mathematical ability (Subject S-3). Data collected through tests, video recordings, field notes, and interviews. Data analysis techniques include data reduction, data presentation, and drawing conclusions. The validity of the data is obtained through validation and triangulation. Subjects with high mathematical ability can solve mathematical problems correctly. Subjects with moderate mathematical abilities are able to solve the given problem but not all indicators on understanding the concept fulfilled. Subjects with low mathematical abilities are not able to solve problems precisely. Research contributes as a new reference to the theory of conceptual understanding.