from traditional teacher-centered and textbook-dominated approaches to new instructional approaches that are student-centered and inquiry-based (Holton 2001). As part of this effort, inquiry-based undergraduate mathematics courses have been examined by some researchers (Kwon et al. 2005;Smith 2005;Rasmussen et al. 2006;Dhaler 2008).Multivariable calculus is a mandatory mathematics subject in most Korean universities, since it prepares students to study at an advanced level in diverse majors such as natural sciences, engineering, and economics. As Rasmussen et al. (2014) point out in the previous issue of ZDM, however, there is a shortage of studies that go beyond basic topics of calculus into areas such as multivariable calculus and differential equations. Also, there is a lack of instructional tasks developed for inquiry based learning (IBL) and a lack of research dealing with classroom interactions and the instructor's role in multivariable calculus teaching/learning. Accordingly, if an instructor wants to apply an inquirybased approach to a multivariable calculus course, he or she might face the difficulty of preparing suitable tasks to cultivate students' knowledge and find that students need to spend a long time at inquiry.In order to overcome this difficulty and to develop iteratively an inquiry-based multivariable calculus course, this study attempts to transform the curriculum and instruction by introducing an innovative instructional model to cover the entire curriculum with inquiry-based mathematical activities. In the fall semester of 2013, a multivariable calculus course for first year students majoring in mathematics education was organized as a flipped classroom at a university in Seoul, Korea. The flipped classroom enabled instructors' explanatory lectures of great amounts of content to be replaced by online video clips in order to assign more time to student inquiry during the face-to-face offline Abstract In this study, researchers design and implement an inquiry based multivariable calculus course in a university which aims at enhancing students' argumentation in rich mathematical discussions. This research aims to understand the characteristics of students' argumentation in activities involving proof constructions through mathematical discussions, and to derive instructional design principles in relation to three sites of intervention: instructional design, classroom interaction, and the instructor's role. Over the course of 14 weeks, 18 freshmen mathematics education majors participated in this study. Multiple sources of data were collected, students' reasoning in the classroom discussions were analyzed within the Toulmin's argumentation structure, and the instructional interventions were gradually revised according to the iterative cyclic process of the design research. The students' argumentation structures presented in the classroom gradually developed into more complicated forms as the study progressed, and the researchers conclude that the interventions were effective at improving student...