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Justification skills should be fostered in teaching mathematics as they are highly related to effectiveness in problem solving. However, pre-service mathematics teachers do not always have the necessary skills in using mathematically convincing language and justifying their solutions. Thus, they should be introduced to a variety of instructional materials and tasks that may contribute to their justification skills. In this study, researchers investigated effect of different learning environments, including dynamic geometry environments and physical instructional materials, on 139 first year pre-service mathematics teachers’ level of justifications. The participants were formed into two treatment groups: the physical instructional materials environment group and the dynamic geometry environment group. In this non-equivalent experimental group design study, the data was collected via pre- and post-tests. No statistically significant difference was found between the treatment groups. The number of mathematically convincing arguments increased at the end of the treatments. The study shows that the use of inquiry-based activities in dynamic geometry and physical instructional material environments might have helped learners improve their level of justifications in geometry.