Students need to have mathematical reasoning abilities so that they can not only solve mathematics but also understand the essence or meaning in mathematics. However, each student has different reasoning abilities. Therefore, the researcher was interested in knowing mathematical reasoning abilities in the material of two-variable linear equations using APOS theory. This study aims to describe students' mathematical reasoning abilities using the stages of the APOS theory. The research subjects were students of grade VIII H. There were6 students selected according to their level of achievement in the class. Sampling was done using purposive sampling. Tests and interviews were used to collect the data. From the data description, it was found that the students with the high achievement of the APOS theory have been good in their mathematical reasoning abilities on a two-variable linear equation system. Students with moderate achievement in the APOS theory have been quite good on their mathematical reasoning abilities of the two-variable linear equation system material. The students with the low achievement of the APOS theory have not been very good in their mathematical reasoning abilities on two-variable linear equation system.