© 2020, Ozgen Korkmaz. All rights reserved.The aim of this research is to understand how the values and beliefs of science teachers reveal in the classes where STEM activities are applied. For this purpose, a researcher and two science teachers applied STEM activities developed by the researcher for 14 weeks in science applications classes. The science teachers and the researcher work in socio-economically different districts of Ankara, Turkey. In this study, one of the qualitative research methods, self-study design was used. Self-study is a research design that contributes to the teacher's understanding of himself/herself, questioning own practice and professional development by becoming a researcher. Data sources are the life stories of the researcher and the participating teachers, in-class video recordings and post-lesson diaries. The obtained data were analyzed by narrative and content analysis. It was understood from their life stories that environmental and socio-cultural factors were effective in science teachers' choosing their profession. It was seen that there were internal and external factors affecting the values and beliefs of science teachers. As a result, it was seen that the researcher improved himself in terms of classroom management in the classes where STEM activities were applied, and that there was a close relation between values and beliefs and their practices.