This study focused on two sets of primary mathematics textbooks in Taiwan, analyzing the unit contents about cylinders and cones. It used learning trajectories to design curriculum and arrange teaching activities for teaching experiments, examining the learning performances of two students at different levels of learning. The study aimed to understand the problem-solving strategies of the students and their myths, if any, in the learning process through the reflective diary of the teacher, for improving teaching effectiveness as well as helping students build a complete and systematic conception of cylinders and cones. Students are expected to have complete thinking in solving such problems, and have the ability to correctly solve mathematical problems.