Students with low functioning autism spectrum disorders (ASD) are students who experience interaction and communication disorders with low intelligence levels. This disorder causes obstacles in learning in class, they have difficulty understanding concepts. Geometry is a mathematical concept that is very relevant to everyday life. This study aims to describe the profile of ASD student's understanding of basic geometric concepts, especially rectangles and squares. The subject in this research is a elementary school student with low functioning ASD. Data was revealed through task-based interviews using instrument of understanding the concept of geometry. The profile of understanding the concept of geometry, devising interpreting, exemplifying, classifying, summarizing, inferring, comparing, and explaining. Some characteristics of autism was very perceptible during understanding the concept of geometry. Subject can interpret the geometry concepts into real objects that are around him. Subjects can provide examples and non-examples of geometric concepts by mentioning the objects around them. By using the media manipulation of geometric shapes, the subject can classify geometric shapes. The subject can identify the characteristics of the concept even by using very simple sentences, but cannot generalize it in the form of a definition. Subjects can compare a real object in the form of a square with a real object in the form of a rectangle. But the subject cannot explain why the object is square or rectangular.