The main purpose of this study was to investigate the number sense performance of 3rd-graders in Taiwan, and to diagnose areas of weakness or deficiency in number sense development. A total of 808 3rd-graders participated in this study. The results indicated that these students did not perform well on each of the five number sense components (correct rates approx. 34%), and they appeared worst on the performance of "Judging the reasonableness of computational results". Boys and girls did not show any appreciable difference in their ability to solve number sense problems. The importance of number sense should be highlighted both by teachers and in textbooks and more time and opportunity provided for students to work on this type of exercise at lower grade levels. This would require that "drill and practice" exercises in mathematics should not indeed be over-taught, and the teaching of number sense to children should begin as early as possible. (Contains 8 tables.)