This study aims to contribute to understanding the scientific argumentation of secondary students in tasks involving different levels of openness in inquiry and the abstraction levels of the scientific knowledge. Thirtyseven students in the seventh grade participated in argumentation regarding photosynthesis. The task characteristics were classified based on four levels of openness in inquiry and four levels of knowledge abstraction. The students’ process of knowledge construction and their argumentation were then analyzed. The results of the study demonstrate that there were differences in task characteristics in actual science classroom depending on teacher’s practice and students’ understanding. There were differences in the students’ argumentation processes according to the level of openness in inquiry. When the students participated in tasks with a high level of openness in inquiry, they felt that they played a central role in the construction of knowledge and were able to evaluate and compare others’ arguments. Accordingly, as cooperative knowledge construction was accelerated, with students producing counter-arguments and rebuttals sequentially within their groups, the students revealed a high level of understanding. Students’ argumentation processes also differed according to the abstraction level of scientific knowledge in the tasks. Although students were greatly challenged and produced a high level of argumentation in the tasks with an abstraction level appropriate to students’ cognitive levels, they felt extremely bored and produced a low level of argumentation in the task with a low abstraction level. By providing a better understanding of task characteristics in scientific argumentation in science classrooms, this study will contribute to supporting students’ participation in more productive argumentative practices.