The key purpose of this study was how we taught the use of benchmark strategy when comparing fraction for fifth-graders in Taiwan. 26 fifth graders from a public elementary in south Taiwan were selected to join this study. Results of this case study showed that students had a much progress on the use of benchmark strategy when comparing fraction after the experiment. This indicated that an experienced teacher played a key role and knew how to help their children lean the use of benchmark appropriately when comparing fraction.