The aim of this study is to compare professional identity of pre-service mathematics teachers in pedagogical formation certificate program and undergraduate teacher education program in the context of pre-service teachers' stories of becoming a mathematics teacher. A total of 113 pre-service elementary mathematics teachers participated in the study. 58 of them were enrolled in undergraduate teacher education program and 55 of them were enrolled in pedagogical formation certificate program. Demographic questionnaire and reflective essays on pre-service teachers' stories of becoming a teacher were used as data collection tools. Demographic questionnaires were analyzed descriptively and pre-service teachers' writings were analyzed using content analysis. Data analysis indicated that two groups had similarities in reasons for why they chose teaching as a profession and mathematics as a subject for teaching. Their earlier teaching experiences in high school and their role model teachers were the most observed reasons for why they chose teaching as a profession. The main difference between the two groups was their determination for aiming to be a teacher. Participants in the undergraduate teacher education program decided to be a teacher in high school while some of the participants in the other group decided to have a teaching certificate just in case they might need it in the future. Interest and success in mathematics were the most important factors for choosing mathematics as a subject for teaching in both groups.