The purpose of this study was to examine the potential interrelationships between teachers' mathematical knowledge for teaching (MKT) the function concept and their students' learning outcomes of this concept. Data were collected from two teachers teaching in a vocational high school and their students through a function concept test for teachers and students, follow-up interviews with teachers, and classroom observations. Findings indicated that teachers' MKT and students' learning outcomes were related to a degree, but this relationship was not straightforward. The data suggested that the teachers' knowledge influenced the quality of their instructional practices, and the instructional practices played a mediating role in student learning. Different factors contributing to student learning were also evident.