Teachers are increasingly expected to incorporate Project-based learning (PBL) pedagogy to teach science. While teachers' definitions and challenges in the use of PBL are widely studied, very few studies focus on the development of a teacher's knowledge of the use of PBL over multiple projects. The current research is an in-depth case study aimed at studying the development of a Finnish class teacher's knowledge on the use of PBL to teach science by using pedagogical content knowledge (PCK) as the lens to study teacher's knowledge. Data was collected in the form of interviews and classroom observation over eight weeks. Concept maps and critical incident analysis were used to analyse the data. The findings show that reflecting on student learning, use of learning materials and unexpected results in science experimentation contributed to the development of the teacher's knowledge of student understanding, assessments, science curriculum, and orientation to science teaching. The teacher's knowledge of student understanding and assessments were closely related and further led to the development of her knowledge in instructional strategies for the use of PBL to teach science.