This study aims to: (1) find out the misconceptions experienced by students in the material of similarity and congruence by using three-tier diagnostic tests based on open-ended questions. (2) find out the large percentage of students' misconceptions on the similarity and congruence material using a three-tier diagnostic test based on open-ended questions. The research method used was descriptive qualitative. The data collection instruments used were clinical interviews and three-tier diagnostic tests based on open-ended questions. This study was conducted in the SMP Negeri 2 Palimanan. The research subjects were selected using purposive sampling techniques so that 33 students were selected from 330 students. The results of the analysis of the three-tier diagnostic test based on open-ended questions showed that (1) the misconceptions that occurred in the material of similarity and congruence of 2D shapes included pure misconceptions, false positives, and false negatives. (2) the percentage of misconceptions experienced in the material of similarity and congruence 2D shapes as a whole was 50.2% with a large percentage of pure misconceptions 32.4%, false-positive 15.6%, and false-negative 2.2%