Argumentation is recognized as a significant aspect of science education for the development of students' scientific literacy, and the science teacher is the key factor in organizing argumentative discourse in the science classroom. Composing argumentation in the classroom requires teachers to not only acquire the basic understandings and skills of argumentation themselves, but also to develop strategies for effective implementation of argumentation in the classroom. The purpose of this paper is to examine pre-service teachers' understanding and practice of argumentation in science teaching. Three pre-service science teachers participated in this study. Their understanding and practice of argumentation in the classroom were examined at the beginning of their teaching practice in terms of three aspects: a) understanding of argumentation, b) ability to compose argumentation; and c) practice with regard to developing argumentation in the classroom. Class observations and interviews were conducted to collect data. Two models were used to analyze the quality of the argumentation and the question types used in the science lessons given by these pre-service teachers. It was found that they had limited understanding of argumentation, and their abilities to compose scientific argumentation were also weak. Corresponding to their limited understanding and skills of argumentation, the questions they raised in the classroom were also not likely to stimulate argumentation. Thus, little argumentation took place in their classrooms. (Contains 6 tables, 1 figure and 1 footnote.)