The purpose of this study is to determine pre-service mathematics teachers' difficulties in constructing proofs and to investigate the changes in their proving skills after teaching activities that they experienced proving and interacted with group discussions. The study was conducted with six fourth grade secondary mathematics teachers. Pre-service teachers' proving skills were investigated with respect to conditional statements and three proof methods; direct proof, proof by contradiction and proof by contraposition. In this qualitative study, teaching experiment method was used. As a result it can be concluded that students' proving skills developed at different levels and they overcome some of their difficulties. Based on the results of the study suggestions are made for the instruction methods of undergraduate mathematics courses.