This study examined the features of peer argumentation in middle school students' scientific inquiry. Participants were two boys and six girls in grade 8 of a middle school in Seoul, Korea. Students engaged in open inquiry activities in small groups. Each group prepared the report for peer review and then, during the peer discussion, presented their inquiry results while another group acted as critics, in a way similar to conference presentations by scientists. This study's data sources included audio- and video-tapes of discussions, copies of student reports, questionnaires completed by the students and transcripts of interviews with the students. It was found that the critical peer discussion in general proceeded through the following four stages: Focusing, Exchanging, Debating and Closing. In addition, 75.6% of evidence used in students' arguments was personal evidence and students used various cognitive and social strategies in the critical discussion. For an effective critical discussion, making good use of the Focusing Stage was found to be important factor. Students improved their interpretation and methods of experiment during the argumentation process and this feedback made the inquiry circular. Finally, we identify a model of argumentative scientific inquiry as an open inquiry that has the key components of authentic scientific inquiry.