The objective of this study was to explore whether elementaryschool teachers use course materials in a different manner afterlearning how to use learning trajectories (LTs). The researchquestions concern the setting of teaching objectives for the unit ofperimeter and area in mathematics, the speculations and analysis ofmathematical concepts with regard to the LTs of perimeter and area,adjustments in learning assignments, the use of representation toassist students and the means of explaining problem-solvingstrategies to students. The subject of this study was a teacher servingat a public elementary school in Central Taiwan. We adopted aqualitative approach in analyzing data from interviews with theteacher and the reflections in the teacher’s journal. The findingsindicate that before the teacher learned how to use LTs, the learningobjectives were focused on strengthening the arithmetic of thestudents, and the speculation of student LTs primarily involvedcalculating and applying the formulas for the perimeter and area ofrectangles. However, after learning how to use LTs, the focus shiftedto gauging student understanding on the relationship among shapeelements. The LT was therefore modified to the following procedure:measuring and calculating the length of segments, understanding theelements composing rectangle shapes, understanding the perimeterformula of rectangles and how to use it in problem solving,understanding the meaning of 1 square centimeter, understanding theelements composing the area of rectangle shapes, and understandingthe area formula of rectangles and how to use it in problem solving.The results of this study provide a clear and successful example ofhow a teacher employs curriculum materials and what theemployment includes.