Science, technology, engineering, and mathematics (STEM) education has been an integral part of many countries' educational policies. In last decade, various practices have been implemented to make STEM areas valuable for 21st century generation. These actions require reconsideration of both pre- and in-service teacher education because those who touch students in and out of the class settings are the teachers. In this respect, this phenomenograhic study aimed to investigate how STEM conceptions of pre-service chemistry and mathematic teachers' (N = 38) evolve as they work together through a STEM module, Collaboratively Learning to Teach STEM (CLT-STEM), depicted by the posters they prepared in groups before and after the implementation. The posters were analyzed with respect to two aspects; conceptions of STEM as a whole, and conceptions of each STEM area individually. For the conceptions of STEM, the codes emerged from data were used; whereas for the conceptions of each STEM area, definitions of these areas were referred. The results of the analyses revealed that the STEM conceptions of the majority of pre-service chemistry and mathematics teachers improved from a lower to a higher level of conception. In terms of the conception of each STEM areas individually, pre-service teachers refined their conception of each area by representing less detail, rather a more comprehensive and integrated view. Therefore, this study may encourage implementing STEM education in preservice science and mathematics teacher education.