Understanding concepts is one of the main goals in mathematics learning. The purpose of this study was to describe the understanding of quadrilateral concepts of junior high school students based on the APOS theory (Action, Process, Objects, and Schemes). A qualitative approach with descriptive explanations was used in this study. The process of understanding the concept begins with observing students by determining one class to be given a mathematical ability test (TKM) to determine students' mathematical abilities and GEFT (Group Embedded Figures Test) which functions to determine groups of students who have the Independent Field (FI) and Field Dependent (FD) cognitive style. The results of the tests of mathematical abilities were analyzed to show the different abilities of students and the GEFT results were analyzed in the presence of students who had cognitive styles FI and FD. Furthermore, it was found two students with the same gender, had equal abilities, and each had a cognitive style of the Independent Field and Dependent Field. The research subjects who were obtained then given quadrilateral concept tasks (TKS) and given interviews. The TKS results were analyzed and the results of the interviews were also analyzed showing that the understanding of students' quadrilateral concepts based on the APOS theory there were differences between students with the Independent field cognitive style and students with field dependent cognitive styles with detailed and thorough descriptions of the results of this study.