In this study, it is aimed to reveal what kind of relationship exists between prospective teachers' thinking structures and their level of writing proofs. This study was carried out as a case study. The study was carried out with 82 prospective teachers enrolled in the Department of Secondary School Mathematics Teaching of a state university located in the Eastern Black Sea Region of Turkey. The first data collection is the mathematical processing instrument developed by Presmeg (1986). The second data collection was developed by the researcher and consists of five proofs that are also covered in the curriculum about the geometric shapes circle and circular region. As a result, when the thinking structures of the participants are examined, it can be understood that harmonic thinking is the most common thinking type among the study participants. When the proof-writing levels in this study are looked at, it becomes obvious that most of the prospective teachers are weak in terms of this skill. In this study, the participants' proof-writing levels and thinking structures were found to be correlated.