This study investigated the effect of scaffolding computer-mediated discussions to improve moral reasoning and argumentation quality in pre-service teachers. Participants of this study were 76 teaching education students at a Turkish university. They were divided into three groups: (1) a computer-supported argumentation group; (2) a computer-mediated discussion group; and (3) a control group. Participants in the computer-supported argumentation group were instructed in argumentation, and were provided with note starters and graphical argumentation tools. The computer-mediated discussion group, however, was engaged in unstructured interaction on the Moodle forum, a free popular learning management system having a threaded discussion forum. The control group did not receive any instruction and neither did they participate in any discussions. As for the results, the computer-supported argumentation group outperformed the control group, but not the computer-mediated discussion group on DIT score and argumentation quality. Thus, it was concluded that both giving instruction on argumentation and appropriately designing the interfaces of computer-mediated discussion environments can enhance argumentation quality in students' writings and also their moral reasoning.