The purpose of this study was to use calculator-assisted instruction to help two fifth-grade low-achievers learn number sense. The research process includes three stages: (1) pre-test interview to detect what kinds of number sense the students did not have; (2) calculator-assisted instruction to help them develop number sense; and (3) post-test interview to examine if their number sense had improved. The results showed that students successfully developed two kinds of number sense abilities they did not have in the pre-test interview. However, they failed to develop the number sense ability of using a benchmark number to make an estimation. Generally, this study illustrated that these two low-achievers could learn number sense from calculator-assisted instruction.