The main purpose of this research is to determine mathematics teachers’ diagnostic competency levels. Diagnostic competence, which is described as the ability to understand and analyze student thinking, has been examined in four levels through the components of teachers’ general knowledge on learning processes and their skills at considering, scrutinizing, and interpreting student thinking. The research model is based on a case study, and the participants consist of three elementary mathematics teachers. Teachers’ lessons were observed for 17 weeks, and data has been collected through (a) observation notes, (b) video recordings, (c) written documents, and (d) interviews with teachers. According to the findings, the teachers have Level-2 diagnostic competences due to their limited general knowledge on learning processes and scrutiny skills. Additional findings are as follows: (a) While teachers have been defined as having Level-2 diagnostic competences in the classroom, lower and higher levels seem able to emerge in environments that used prompts. (b) The limitations of teachers’ general knowledge on learning processes can also weaken other diagnostic competency skills. (c) Teachers care more about students’ errors and mistakes when diagnosing and do not feel the need to diagnose ideas that provide correct results.