The purpose of this study was to investigate science teachers' understandings about scientific argumentation in the classroom. Seven structured interview protocols were developed, asking the definition of scientific inquiry, the differentiation between scientific inquiry and hands-on activity, the opportunity of student argumentation, explicit teaching strategies for scientific argumentation, the critical example of argumentation, the criteria of successful argumentation, and the barrier of developing argumentation. The results indicate that there are differences and similarities in understandings about scientific argumentation between two groups of middle school teachers and upper elementary. Basically, teachers at middle school define scientific inquiry as the opportunity of practicing reasoning skills through argumentation, while teachers at upper elementary define it as the more opportunities of practicing procedural skills through experiments rather than of developing argumentation. Teachers in both groups have implemented a teaching strategy called "Claim-Evidence Approach," for the purpose of providing students with more opportunities to develop arguments. Students' misconception, limited scientific knowledge and perception about inquiry as a cycle without the opportunity of using reasoning skills were considered as barriers for implementing authentic scientific inquiry in the classroom.