This study examines the impact of teachers’ subject matter knowledge on students’learning. The mathematical content deals with rational number as fractions andproportion. The study includes pre- and post-tests from 99 students, classroomobservations, students’ written solutions and interviews with 48 selected studentsafter the post-tests. Findings from this study show that the impact of teachers’ subjectmatter knowledge and ability to identify the objects of learning, and apply this inteaching, strongly influenced the development of students’ conceptual learning aboutfractions and proportion.