This research aimed to explore the development process of group argumentation on socio-scientific issues based on the interactions between group members. Using the data from nine pre-service elementary school teachers, two groups` argumentation on six socio-scientific issues was qualitatively described based on the Grounded Theory approach. Through content analysis on argumentative discourse, the interactions among group members and their sophistication of group arguments were described. As the results of this study, it was found that the higher level of argumentation showed both active engagement and deep collaborative reasoning. Based on these results, it was discussed how can we, science educators, encourage argumentations on socio-scientific issues in school science classrooms.