Assessing elementary students’ performance on number sense provides early predictors and support for interventions in teaching and learning number sense before children fall seriously behind in school. There is no study that concerns the differences between Hong Kong and Taiwan students’ performance on the number sense three-tier test. The purposes of this study were to examine the differences of Hong Kong and Taiwan students’ performance on the number sense three-tier test and the possible misconceptions. A total of 125 fifth graders, 942 sixth graders from Hong Kong, and 819 fifth graders from Taiwan participated in this study. Results show that there are significant differences on the performance of first two-tier test and confidence between the fifth graders of HK and Taiwan, sixth graders of HK and fifth graders of Taiwan, respectively. Strong misconceptions and key differences among the HK fifth graders, HK sixth graders, and Taiwan fifth graders were found and discussed.