We focus on students' mathematical reasoning in a technology-enhanced collaborative learning environment. We adopt a dialogical approach to analyze students' mathematical reasoning. The participants of this study include six middle school students. The data consist of participants' written productions, dynamic materials, and the transcriptions of the participants' discourse. The analysis shows that the integration of dynamic mathematics software into the ACODESA method contributes to their collective mathematical reasoning productively. The use of dynamic mathematics software as mediational artefacts and productive discussion as semiotic mediation are also required to enhance the participants' both structural and process aspects of mathematical reasoning. The mediational role of dynamic mathematics software also helps them to make dynamic connection between mathematical reasoning and proving. In addition, participants' representations evolve in the technology-enhanced learning environment and this evolution contributes to the development of mathematical reasoning.