A pivotal information about the structure of students' comprehension underlying from which the knowledge is gained can be identified through the mental model. This study aimed at describing the six levels of students' mental model in comprehending the concept of the integer. The subject of this research was 40 students consisting of 20 students from the second graders of primary school and 20 students from the third graders of primary school. The subject was taken from two classes in order to meet the whole levels of the mental model. The subject of the research was given a test to know their comprehension of the concept of the integer. From the subject, there were 6 students who were interviewed because they have already fulfilled the six levels of the mental model. To guarantee the validity and reliability of the research, the triangulation technique was done by comparing the video recording of the interview with the results of the test. The results reveal that there are six levels of students' mental model in comprehending the concept of integer, i.e. pre-initial mental model, initial mental model, the transition 1, synthetic mental model, the transition 2, and formal mental model.