One of the essential cognitive aspects of learning mathematics is the ability to understand concepts. This study describes the ability to understand students' mathematical concepts based on brain dominance. The type of research used is descriptive qualitative. The research subjects were high school students. The instruments used were tests, questionnaires, and interview guidelines. Data nalysis techniques consist of data reduction, presentation, and concluding. The results showed that students left brain dominant student have a good mathematical concepts understanding ability, provide clear and detailed information, provide examples with different shapes or arrangements, use the other symbols than numbers, and they are confident that they understand the concepts. Meanwhile, right-brain dominant students have a fairly good mathematical concepts understanding ability, tend to convey information randomly using their own language, provide examples with similar shapes or arrangements, and are less sure that they understand the concept.