Students' difficulties in understanding the circle concept are important issues to be resolved by starting a more in-depth research first about identifying and finding out what difficulties deaf students experience in understanding the circle concept. The identification results can be used by the teacher to help students overcome their difficulties and as material for consideration in carrying out remedial teaching. The aim of this study was to identify how the difficulties experienced by deaf students in understanding the circle concept. The research used was a mixed-method with a sequential explanatory design. The data to be collected first was quantitative data from understanding test then analyzed. Based on these results the researcher collected qualitative data by observed the results of students' written answers and other external representations, then interviewed students to help explain in more detail the results of the initial quantitative data. The subjects in this study were nine deaf students at SLB Negeri 2 Bantul. Data analysis was carried out quantitatively and qualitatively. Based on the research results, knew that the mean score of understanding the concept of circles in deaf students is less than 60 or it hasn't reach the minimum completeness criteria. Most of the students' test answers were represented visually by drawing. From the research results, also knew that the difficulty of understanding students is related to the background of total communication barriers due to their hearing loss. The results of the identification of the difficulty in understanding the deaf students are difficulties in processing information, language difficulties, organizational difficulties, attention difficulties, output difficulties, difficulties with multiple tasks and mathematics anxiety.