This study aims to determine students' difficulties in understanding the concept of polygons in elementary schools. This research is qualitative research with a case study method. The subjects of this study were 30 grade 5 elementary school students from two different schools in the city of Bandung. The instruments used are test and non-test. The technique of tests is asking some questions about polygons, while the non-test is in the form of interviews. The data collected were analyzed using the three stages of the Miles and Huberman model, including reduction, presentation, and conclusion. The findings in this study indicate that students have difficulty understanding the concept of polygons, namely difficulties in identifying polygons properties, polygons rules and regulations, and determining polygons' names. One of the things that teachers can do to follow up on the problems from these findings is to design learning based on didactic situations that are appropriate and according to the problem faced by students.