The purpose of this study was to examine students' understanding of argumentation when talk and writing were provided as learning tools, as well as to explore how talk and writing can best support students' construction of scientific knowledge. Most current studies have examined discourse patterns over a short interval of only a few class periods or compared only the students' initial and final products to assess the quality of their argument structure. Few studies have examined how students develop their understanding of argumentation over time and how their understanding might result in overcoming those challenges. Moreover, talk and writing have been offered as two critical learning tools to support students' argumentative practice. So far, few studies have explored how those two learning tools could be combined to better support students in constructing scientific knowledge. The research questions that guided this study were: (1) How do students develop an understanding of the components of argumentation for public negotiations over time when participating in an argument-based inquiry classroom? (2) In what ways do talk and writing support scientific knowledge construction in an argument-based inquiry classroom? This sixteen-week study was grounded in interactive constructivism and utilized qualitative design to identify students' understanding of argumentation, trace their learning trajectories, examine potential use of the combination of talk and writing, and analyze the cognitive processes involved when talk and writing were used as learning tools. Due to the lack of studies that focus on the elementary level, this study was conducted in a fifth-grade classroom that used the Science Writing Heuristic (SWH) approach with 22 students participating. Six students were selected for interviewing intensively. Multiple sources of data were collected, including classroom observations, semi-structured interviews, students' writing samples, and the researcher's field notes. To strengthen the interpretations, data analysis was conducted using three different approaches: (1) the constant comparative method, (2) the enumerative approach, and (3) in-depth analysis of knowledge construction trajectory (KCT) episodes. The results showed that as fifth-grade students had more opportunities to practice, they could develop a more sophisticated understanding of argumentation, use talk and writing as learning tools to negotiate their ideas with peers, engage in more complex cognitive processes, and take ownership for their learning in science. Three major findings are discussed: (1) increased understanding of argumentative components in public negotiations, (2) increased ability to craft written arguments, and (3) five patterns in the use of talk and writing for knowledge construction and cognitive processes. The findings have informed theories about argumentative practice, the use of language as a learning tool, and science learning from six aspects: (1) understanding of argumentation, (2) ability to craft written arguments, (3) use of talk and writing, (4) cognitive processes, (5) meaning of negotiation, and (6) methodology consideration. This study provides insights into the design of an argument-based environment in which students can develop successful argumentative practices. A long-term professional development program in the support of teachers implementing argument-based inquiry is suggested.