This paper outlines some preliminary findings from a one-year problem-based learning (PBL) intervention in a Taiwanese elementary mathematics classroom. PBL stands within the philosophy of social constructivism, which emphasises that learning is a social process, not a product. In general, it involves three main characteristics. The first is that the content is organised as a problem or a series of problems, rather than in textbook form. The second is that students work as groups to solve problems and learn from small group collaborative interactions rather than being taught by the teacher. The third is the student-centred situation: students are not in classrooms waiting for their teachers to give them instruction, but are there to construct knowledge and to establish a new level of knowledge. The PBL process in this study generally followed three steps. The first step was to launch a problem. Miss Lee introduced the problem/activity to students. The second step was to solve the problem through group/paired discussion. During this stage, Miss Lee circulated among groups at all times. The third step was to conduct a whole class discussion about the problem being investigated. Students articulated their solutions and gave/received feedback to further reason about the problem. Some preliminary findings from this one-year PBL intervention are presented in this article.