Pre-service secondary mathematics teachers’(PSMTs) understanding and ability of constructing a proof is not only importantfor their own learning process, but also important for these PSMTs to helptheir future students learn how to do proofs. Therefore, this study is focusedon and explains PSMTs’ behaviors that they revealed throughout the provingprocess of a proposition. In this qualitative case study, the participants werefifteen volunteer PSMTs from a public university in Turkey. The participantswere given a proposition which they were asked to think aloud and prove it onthe blackboard. The findings of this study show that PSMTs’ behaviors andthoughts regarding the given proposition were limited. In detail, PSMTs haddifficulties in application of mathematical language and notations,understanding the meaning of the given proposition, knowing where to getstarted on a proof, using examples efficiently, using appropriate and efficientmethods to construct the proof, and defining logical structures of theproposition to construct the proof.