The research background had the importance of ability to understand mathematical concepts in learning mathematics in college. This study aimed to analyze the students’ ability in understanding mathematical concepts in terms of high, medium, and low abilities. The research subjects were ten people who are students of the mathematics education study program. The research method used was descriptive qualitative and the research instrument used was a test of the ability to understand mathematical concepts is a test of the ability to understand mathematical concepts in the course of Number Theory. The results obtained in high-ability students were the students can answer all questions about understanding mathematical concepts and there was only one person who did not have complete answers in questions number three and four. In medium-ability students the questions number one, two, and three can be answered well and about numbers four two respondents answered but the procedure was wrong and two people did not answer. In low-ability students the questions number one, two and three can be answered correctly and for question number four all respondents did not answer it.