This is a qualitative research that aims to describe eight graders' ability in understanding mathematical concepts, especially on the material entitled Relation and Function, in an Islamic high school named MTs Al-Ittihaad Ma'arif NU 1 located in Purwokerto Barat. Data obtained were analyzed through three techniques; data reduction, data presentation, and conclusion. While the sample was taken by employing purposive sampling technique. In this research, students were grouped into three groups (high, moderate, and low) based on the result of the mid-term test in the odd semester of academic year 2017/2018. Instruments used in research were written test, interview, and documentation. While the triangulation technique was employed to test the validity of the data. Research results revealed that students who got high learning outcome were able to understand mathematical concept on Relation and Function material well. It was proven since they could master all indicators of the ability in understanding mathematical concepts included restating a concept, classifying objects based on certain natures corresponding with a concept, giving examples of a concept, presenting a concept in various forms of mathematical representation, and utilizing as well as choosing certain procedure or operation. Students with moderate achievement had medium ability in understanding mathematical concepts. They were only able to master some indicators of the ability in understanding mathematical concepts; restating a concept and classifying objects based on certain natures corresponding with a concept. While students who got low achievement had poor ability in understanding the concepts. They only could master one of the indicators that was classifying objects based on certain natures corresponding with a concept.